it is well - known that 
 self - assembly of ab diblock copolymers occurs 
 in appropriate selective solvents . this is typically conducted in dilute solution ( < 1% ) and usually 
 involves post - polymerization processing methods such as a solvent 
 switch , a ph switch , or thin film rehydration . the 
 precise diblock copolymer morphology that is generated mainly depends 
 on the relative volume fractions of the two blocks , because this dictates 
 the so - called packing parameter . the 
 most commonly reported copolymer morphologies are spheres , worms / cylinders , 
 or vesicles . however , there are many studies 
 describing spheres and vesicles , while relatively few papers are focused 
 on worms / cylinders . presumably , this is simply because the worm / cylinder 
 morphology occupies the narrowest region of the phase diagram . nevertheless , there has been 
 considerable recent activity focused on the production of block copolymer 
 worms / cylinders , often based on semicrystalline core - forming blocks . we and others have recently shown that polymerization - induced self - assembly ( pisa ) 
 offers a potentially decisive option for the formation of diblock 
 copolymer nano - objects . for example , spherical , 
 worm - like , or vesicular block copolymer morphologies can be efficiently 
 generated directly in either water , alcohol , or n - alkanes at relatively high 
 solids ( up to 25% ) during the synthesis of the copolymer chains . the worm phase is particularly interesting , because such particles 
 can either act as thickeners or form free - standing gels , depending 
 on the copolymer concentration . recently , blanazs et al . have shown 
 that a semi - concentrated aqueous dispersion of methacrylic diblock 
 copolymer worms undergoes a worm - to - sphere morphological transition 
 on cooling from 20 to 5 c . this results in thermo - reversible degelation , which allows 
 facile sterilization via cold ultrafiltration because this protocol 
 removes any micrometer - sized bacteria that may be present . such worm 
 gels are also highly biocompatible , and both the gel strength and 
 the critical gelation temperature can be tuned ; thus they are expected to have biomedical applications 
 in the context of cell storage media . in the present work , we 
 revisit a new raft non - aqueous dispersion 
 polymerization formulation that enables a range of poly(lauryl methacrylate)poly(benzyl 
 methacrylate ) [ plma  pbzma ] diblock copolymer nano - objects to 
 be prepared in n - heptane . working at a fixed solids concentration of 20% w / w in n - dodecane , a phase diagram has been constructed that reveals the 
 effect of varying the mean degrees of polymerization ( dp ) of the stabilizer 
 and core - forming blocks on the final copolymer morphology . using n - dodecane in place of n - heptane facilitates 
 studies of the worm gel phase , because the problem of in situ solvent 
 evaporation is avoided . furthermore , we demonstrate that plma - pbzma 
 diblock copolymer worm gels undergo reversible degelation 
 on heating ( see figure 1 ) . transmission 
 electron microscopy ( tem ) , dynamic light scattering ( dls ) , gel rheology , 
 variable temperature h nmr spectroscopy , and small - angle 
 x - ray scattering ( saxs ) are used to characterize this phase transition , 
 and a physical mechanism for worm disintegration is suggested . ( a ) raft synthesis 
 of poly(lauryl methacrylate ) ( plma ) macro - cta 
 via raft solution polymerization in toluene at 70 c , followed 
 by raft dispersion polymerization of benzyl methacrylate ( bzma ) in n - dodecane at 70 c . ( b ) schematic representation of 
 the change in morphology that occurs on increasing the pbzma target 
 degree of polymerization when using a relatively short plma macro - cta . 
 ( c ) thermo - responsive solution behavior exhibited by 20% w / w plma16pbzma37 diblock copolymer nanoparticles 
 in n - dodecane . a free - standing gel is formed at 20 
 c , which becomes a free - flowing solution when heated to 70 c 
 ( see main text for an explanation of this phase transition ) . monomers were purchased from sigma - aldrich 
 ( uk ) and passed through basic alumina prior to use . n - dodecane ( 90% technical grade ) , cdcl3 , and all 
 other reagents were purchased from sigma - aldrich ( uk ) and were used 
 as received , unless otherwise noted . thf and toluene were purchased 
 from fisher scientific ( uk ) , while cd2cl2 and d26-dodecane were purchased from goss scientific 
 ( uk ) . the synthesis of plma macro - ctas has been described 
 in detail elsewhere , and therefore only 
 one representative formulation is briefly discussed here . a 100 ml round - bottomed flask was charged with lauryl methacrylate 
 ( lma ; 30 g ; 118 mmol ) , cumyl dithiobenzoate ( cdb ; 3.22 g ; 11.8 mmol ) , 
 2,2-azobis(isobutyronitrile ) ( aibn ; 387 mg , 2.37 mmol ; cdb / aibn 
 molar ratio = 5.0 ) , and toluene ( 50.4 g ) . the sealed reaction vessel 
 was purged with nitrogen and placed in a preheated oil bath at 70 
 c for 11 h. the resulting plma ( lma conversion = 79% ; mn = 4900 g mol , mw = 5400 g mol , mw / mn = 1.19 ) was 
 purified by precipitation into excess methanol . the mean degree of 
 polymerization ( dp ) of this macro - cta was calculated to be 16 using h nmr spectroscopy by comparing the integrated signals corresponding 
 to the cdb aromatic protons at 7.18.1 ppm to that assigned 
 to the two oxymethylene protons of plma at 3.74.2 ppm . a typical 
 raft non - aqueous dispersion polymerization synthesis of plma16pbzma37 diblock copolymer worms at 20% w / w solids 
 was carried out as follows . benzyl methacrylate ( bzma ; 0.450 g ; 2.55 
 mmol ) , aibn initiator ( 4.50 mg ; 0.027 mmol ; dissolved at 1.0% w / w 
 in bzma ) , and plma16 macro - cta ( 0.30 g ; 1.10 mmol ; macro - cta / initiator 
 molar ratio = 5.0 ) were dissolved in n - dodecane ( 4.0 
 ml ; 3.00 g ) . the reaction mixture was sealed in a 10 ml round - bottomed 
 flask and purged with nitrogen gas for 25 min while being immersed 
 in an ice bath to reduce solvent evaporation . the deoxygenated solution 
 was then placed in a preheated oil bath at 70 c for 16 h ( final 
 bzma conversion = 98% ; mn = 10  800 
 g mol , mw / mn = 1.25 ) . the same diblock copolymer formulation 
 was scaled up for rheological studies . in further syntheses , the mean dp of the pbzma block was systematically varied by adjusting the amount 
 of added bzma monomer under otherwise identical reaction conditions . molecular weight distributions 
 were assessed by gel permeation chromatography ( gpc ) using thf eluent . the gpc setup comprised two 5 m ( 30 cm ) mixed c columns , a 
 wellchrom k-2301 refractive index detector operating at 950  
 30 nm , a precision detector pd 2020 light scattering detector ( with 
 scattering angles of 90 and 15 ) , and a bv400rt solution 
 viscosity detector . the mobile phase contained 2.0% v / v triethylamine 
 and 0.05% w / v butylhydroxytoluene , and the flow rate was fixed at 
 1.0 ml min . a series of 10 near - monodisperse poly(methyl 
 methacrylate ) standards ( mp values ranging 
 from 1280 to 330  000 g mol ) were used for 
 calibration . h nmr spectra 
 were recorded in either cd2cl2 or cdcl3 using a bruker av1 - 400 or av1 - 250 mhz spectrometer . typically 64 
 scans were averaged per spectrum . for variable - temperature h nmr studies , 0.50 ml of a 20% w / w dispersion of plma16pbzma37 worms was diluted to 10% w / w using n - dodecane ( 0.50 ml ) prior to centrifugation at 8000 rpm 
 for 8 h. the sedimented worms were then redispersed with the aid of 
 an ultrasonic bath using d26-dodecane 
 ( 2.0 ml ) to produce a 5.0% w / w dispersion . h nmr spectra 
 were recorded at various temperatures ranging from 25 to 149 c 
 ( 32 scans per spectrum , delay time = 0.10 s ) using a bruker av1 - 500 
 mhz spectrometer . dynamic 
 light scattering ( dls ) 
 studies were performed using a zetasizer nano - zs instrument ( malvern 
 instruments , uk ) at a fixed scattering angle of 173. copolymer 
 dispersions were diluted in n - dodecane prior to light 
 scattering studies at 25 c . temperature - dependent dls studies 
 were performed using the same zetasizer nano - zs instrument , which 
 was equipped with a peltier cell . copolymer dispersions were diluted 
 in n - dodecane and equilibrated for 5 min at 10 c 
 intervals in a 20 c90 c20 c thermal 
 cycle . in both sets of experiments , the intensity - average diameter 
 and polydispersity of the diblock copolymer particles were calculated 
 at a given temperature by cumulants analysis of the experimental correlation 
 function using dispersion technology software version 6.20 . transmission electron 
 microscopy ( tem ) studies were conducted using a philips cm 100 instrument 
 operating at 100 kv and equipped with a gatan 1 k ccd camera . diluted 
 block copolymer solutions ( < 0.50% w / w ) were placed on carbon - coated 
 copper grids and exposed to ruthenium(iv ) oxide vapor for 7 min at 
 20 c prior to analysis . this heavy 
 metal compound acted as a positive stain for the core - forming pbzma 
 block to improve contrast . the ruthenium(iv ) oxide was prepared as 
 follows : ruthenium(ii ) oxide ( 0.30 g ) was added to water ( 50 g ) to 
 form a black slurry ; addition of sodium periodate ( 2.0 g ) with stirring 
 produced a yellow solution of ruthenium(iv ) oxide within 1 min . an ar - g2 rheometer equipped with 
 a variable - temperature peltier plate and a 40 ml 2 aluminum 
 cone was used for all experiments . the loss and storage moduli ( g and g , respectively ) 
 were measured as a function of temperature at a heating rate of 1.0 
 c per minute , a fixed strain of 1.0% , and an angular frequency 
 of 10 rad s so as to assess the gel strength and 
 critical gelation temperature ( cgt ) . during temperature sweeps , the 
 temperature was varied at 5 c intervals , with an equilibration 
 time of 5 min being allowed prior to each measurement . a frequency 
 sweep from 0.1 to 100 rad s was conducted at 20 
 c using a fixed strain of 1.0% and an equilibration time of 
 1 min between each measurement . in all cases , the sample gap was 58 
 m . saxs patterns 
 were collected at a synchrotron source ( esrf , station bm26 , grenoble , 
 france ) using monochromatic x - ray radiation ( wavelength  = 
 0.1 nm , with q ranging from 0.023 to 1.3 nm , where q = 4 sin / is the 
 length of the scattering vector and  is one - half of the scattering 
 angle ) and a 2d pilatus 1 m ccd detector . time - resolved saxs patterns 
 were recorded at a rate of 2 frames per minute during thermal cycles 
 performed on the samples ( heating from 20 c to 160 c at 
 a rate of 5 c min , equilibrating for 5 min , 
 and then cooling back to 20 c at 5 c min ) . the sample temperature was controlled by a heating / cooling capillary 
 holding stage ( linkam scientific instruments ltd . , scattering data were reduced by nika sas macros for igor pro ( integration , 
 normalization , and background subtraction ) and were further analyzed 
 using irena sas macros for igor pro . measurements were conducted on 1.0% w / w and 
 5.0% w / w dispersions of plma16pbzma37 particles in n - dodecane . this afforded low polydispersity 
 plma macro - ctas in high yield with a mean degree of polymerization 
 ( dp ) of 16 , 18 , or 21 ( see supporting information table s1 ) . each macro - cta was formed using cumyl dithiobenzoate 
 as a cta . in all lma polymerizations , the reaction was quenched at 
 73%84% conversion , so as to avoid monomer - starved conditions 
 and hence ensure retention of the raft end - groups . this is a prerequisite for high blocking efficiencies and 
 hence well - defined plma each plma 
 macro - cta had a polydispersity ( mw / mn ) of less than 1.25 , which is consistent with 
 previous studies reporting well - controlled raft syntheses . bzma monomer 
 was polymerized using each of the relatively short , low polydispersity 
 plma macro - ctas ( dp = 16 , 18 , or 21 ) in turn via raft dispersion polymerization 
 in n - dodecane to generate a series of well - defined 
 plma  pbzma diblock copolymers at 20% w / w solids ( see supporting information figure s1 and table s2 ) . 
 in all cases , more than 94% bzma conversion was achieved within 16 
 h at 70 c , as judged by h nmr spectroscopy . tem 
 studies indicated that spherical , worm - like , or vesicular morphologies 
 can be accessed when chain - extending plma16 and plma18 macro - ctas ( see figure 2 ) . however , 
 only spherical morphologies were obtained when using a longer plma21 macro - cta . this indicates that the upper limit degree of 
 polymerization for the stabilizer block to form higher order morphologies 
 ( i.e. , either worms or vesicles ) is relatively low for this plma  pbzma / n - dodecane formulation . we reported similar results for 
 the synthesis of the same diblock copolymer via raft dispersion polymerization 
 in n - heptane at 90 c . in this earlier study , a detailed phase diagram was constructed 
 for a fixed plma stabilizer dp of 17 , with variables being the mean 
 dp of the pbzma core - forming block and the total solids at which the 
 syntheses were conducted . it was shown that lower concentrations ( 15% 
 w / w solids ) typically led to mixed phases , with the formation of pure 
 phases requiring somewhat higher concentrations ( > 17.5% w / w ) . thus , 
 in the present work , we chose to construct a different phase diagram , 
 whereby all syntheses were conducted at 20% w / w solids and the two 
 variables were the mean dps of the plma and pbzma blocks . three plma 
 macro - ctas with dps of 16 , 18 , and 21 were utilized , while the dp 
 of the pbzma block was systematically varied from 20 to 80 ( see figure 2 and supporting information table s2 ) . as anticipated , the worm phase region is relatively narrow 
 ( e.g. , less than 10 bzma units for plma18pbzmax ) . the worms have relatively well - defined 
 mean widths ( 1020 nm ) , but are rather polydisperse in length 
 ( 1001000 nm ) . this suggests that the mechanism of worm formation 
 during the raft polymerization of bzma most likely involves the one - dimensional 
 aggregation and fusion of monomer - swollen spheres . similar observations 
 have been reported for various other diblock copolymer worm systems 
 in the literature . phase diagram 
 constructed for plmax  pbzmay diblock copolymer nanoparticles 
 prepared by raft dispersion polymerization of bzma in n - dodecane at 20% w / w solids using aibn at 70 c ( plma / aibn molar 
 ratio = 5.0 ) . note that higher order morphologies ( worms and 
 vesicles ) can be obtained when using plma16 and plma18 macro - ctas , but not when using the plma21 macro - cta . tem images ( a ) , ( b ) , and ( c ) correspond to examples of the three pure 
 morphologies ( spheres , worms , or vesicles ) , respectively , and the 
 individual block copolymer compositions are indicated within the phase 
 diagram . focusing on the worm phase boundary , 
 we previously reported that 
 plma17pbzmax worms 
 were obtained in n - heptane at 20% w / w solids when x = 5060 . inspecting 
 figure 2 , the worm phase appears to be located 
 within x = 3540 for this hypothetical diblock 
 composition at 20% w / w in n - dodecane . this implies 
 a significant shift in the worm phase boundaries on switching from n - heptane to n - dodecane , which in turn 
 suggests that each n - alkane solvent requires the 
 construction of a detailed phase diagram to ensure reproducible targeting 
 of pure diblock copolymer morphologies . tube inversion tests 
 confirm that the worms form transparent free - standing 
 physical gels at 20 c , whereas the spherical and vesicular dispersions 
 remain free - flowing , low - viscosity fluids ( see supporting information figure s2 ) . presumably , gelation is 
 the result of multiple inter - worm interactions . these worm gels also 
 proved to be thermo - responsive , with degelation occurring on heating 
 the dispersion . the rest of this article is focused on the detailed 
 study of this unexpected thermal transition . variable - temperature 
 dynamic light scattering ( dls ) studies showing 
 the variation of hydrodynamic diameter ( blue  ) and polydispersity 
 ( red  ) for a 0.10% w / w plma16pbzma37 dispersion in n - dodecane on heating from 
 20 to 90 c ( filled symbols ) and on cooling from 90 to 20 c 
 ( open symbols ) . representative transmission electron microscopy ( tem ) 
 images obtained for the same dilute dispersion are shown as insets . these data confirm the irreversible nature of the worm - to - sphere transition 
 that occurs on heating plma16pbzma37 under these conditions . to examine the physical mechanism of degelation , dls 
 studies were conducted on a highly dilute ( 0.10% w / w ) plma16pbzma37 worm dispersion in n - dodecane 
 ( see figure 3 ) . an apparent sphere - equivalent 
 hydrodynamic diameter of 170 nm ( dls polydispersity = 0.37 ) was observed 
 at 20 c , which is consistent with the presence of worm - like 
 particles . however , heating this dilute dispersion resulted in a dramatic 
 reduction in the apparent hydrodynamic diameter , from 167 nm at 60 
 c to around 60 nm at 90 c . the nanoparticle polydispersity 
 at 90 c is also reduced to around 0.14 , which is characteristic 
 of spheres rather than worms . the dimensions of these particles remained 
 relatively constant on cooling to 20 c ( hydrodynamic diameter 
 = 46 nm ; dls polydispersity = 0.13 ) , suggesting that this thermal 
 transition is essentially irreversible when conducted in sufficiently 
 dilute solution . tem studies of the same dilute dispersion conducted 
 before and after the 209020 c thermal cycle 
 confirmed an irreversible change in morphology for 
 the plma16pbzma37 particles ( at least 
 on a time scale of several hours ) . a well - defined worm morphology 
 is observed at 20 c before the heating cycle ( see image ( a ) 
 in figure 3 ) , whereas a predominantly spherical 
 morphology exists at 20 c after the heating cycle ( see image 
 ( b ) in figure 3 ) . presumably , degelation occurs 
 on heating once a critical fraction of worms has been converted into 
 spheres , because this reduction in particle anisotropy inevitably 
 leads to a catastrophic reduction in the number of inter - particle 
 contacts . variation of storage moduli ( g , blue squares ) 
 and loss moduli ( g , red squares ) for a plma16pbzma37 worm gel at 20% w / w during a thermal 
 cycle of heating from 20 to 90 c ( filled symbols ) and cooling 
 from 90 to 20 c ( open symbols ) at a rate of 1.0 c per 
 minute . data were recorded at 1.0% strain using an angular frequency 
 of 10 rad s and an equilibration time of 5 min 
 between each measurement . rheological studies were performed on a representative 
 worm gel ( 20% w / w plma16pbzma37 in n - dodecane ) . at 20 c , g was 
 found to be relatively independent of frequency ( see supporting information figure s3 ) , indicating that the worm 
 gel can be considered a  true additionally , variable - temperature 
 studies were conducted to characterize the thermo - responsive behavior 
 of this gel . the critical gelation temperature ( cgt ) is defined as 
 the point at which the g and g curves cross over . on heating to 47 c , a cgt was observed 
 at approximately 47 c ( see figure 4 ) . the gel had reformed at 20 c , with the g 
 and g curves intersecting at approximately 
 the same cgt during the cooling cycle . however , the final gel strength ( g  87 pa ) was substantially reduced when compared 
 to the original gel strength ( g  
 2300 pa ) , which indicates significant hysteresis under these conditions . it is perhaps noteworthy that the critical temperature ( > 70 
 c ) 
 required for the worm - to - sphere transition suggested by the dls studies 
 shown in figure 3 is somewhat higher than that 
 indicated by the rheological studies ( cgt  47 c ) shown 
 in figure 4 . it is possible that this discrepancy 
 may be simply the result of the differing copolymer concentrations 
 ( 0.10% w / w for dls vs 20% w / w for rheology ) . however , the mean particle 
 size reported by dls will only decrease significantly once almost 
 all of the worms have been converted into spheres , because the scattered 
 light intensity is proportional to the sixth power of the particle 
 dimensions . in contrast , it is likely that only a minor fraction of 
 worms need to be converted into spheres to cause degelation in the 
 rheology studies . thus , these two techniques are understandably sensitive 
 to different stages of the worm - to - sphere transition . systematic 
 dilution of a 20% w / w plma16pbzma37 worm 
 gel in n - dodecane was undertaken to 
 estimate the critical gelation concentration ( cgc ) . this dispersion 
 failed a tube inversion test at approximately 11% w / w , indicating 
 degelation at this concentration . this is significantly higher than 
 the cgc of approximately 34% w / w observed for aqueous diblock 
 copolymer worm gels reported by verber et al . one possible explanation for this unexpected difference is that 
 the aqueous worms may have a significantly greater mean contour length 
 than the worms described herein . initially , worms are observed at 20 c on 
 dilution to 0.01% w / w solids [ see image ( a ) ] . on heating this 20% 
 w / w gel to 90 c , degelation occurs . allowing the hot 20% w / w 
 worm gel to cool from 90 to 20 c ( followed by dilution to 0.01% 
 w / w solids prior to preparing a tem grid ) reveals mainly a worm phase [ see image ( b ) ] , with a minor population of isolated spheres ( see 
 red arrows ) . tem studies of the original 
 20% w / w plma16pbzma37 worm gel ( after 
 dilution of this dispersion to 0.01% w / w 
 using n - dodecane ) reveal a pure worm phase at 20 
 c , as expected ( see figure 5a ) . heating 
 this 20% w / w worm gel to 90 c causes degelation , indicating 
 a worm - to - sphere transition . cooling 
 this concentrated dispersion to 20 c led to regelation . after 
 dilution to 0.01% w / w solids , tem studies confirmed the reformation 
 of worms , albeit with a minor population of spheres ( see figure 5b ) . thus , the worm - to - sphere thermal transition 
 appears to be reasonably reversible for concentrated copolymer dispersions , 
 in marked contrast to the irreversible behavior observed for highly 
 dilute dispersions ( see figure 3 ) . in unpublished 
 work , we have recently observed a similarly strong concentration dependence 
 for aqueous diblock copolymer worm gels / dispersions . presumably , this 
 reflects the highly cooperative nature of the sphere - to - worm transition , 
 which requires the self - assembly of many spheres to form a single 
 worm . such multiple fusion events are much less likely to occur for 
 highly dilute dispersions , at least within normal experimental time 
 scales ( hours / days ) . variable - temperature h nmr spectra recorded 
 for 5.0% 
 w / w plma16pbzma37 diblock copolymer 
 worms in d26-dodecane . variable - temperature h nmr spectroscopy 
 studies were 
 undertaken using d26-dodecane to examine 
 the molecular basis for the worm - to - sphere transition . on heating 
 above 70 c , the aromatic signals assigned to the bzma residues 
 at 6.77.4 ppm become increasingly intense relative to the 
 two oxyethylene protons assigned to the plma block at 4.0 ppm ( see 
 figure 6 ) . this spectral change proved to be 
 reversible on cooling and suggests solvent plasticization of the core - forming 
 block at elevated temperature . similar effects have been reported 
 by price and co - workers for polystyrene - core diblock copolymer micelles 
 dispersed in n - octane . the same workers also reported an irreversible worm - to - sphere 
 transition on heating a dilute dispersion of polystyrene however , no attempt was made to study this thermal 
 transition for relatively concentrated copolymer gels / dispersions , 
 where the change in morphology becomes more or less reversible ( as 
 in the present work ) . thus , these variable - temperature h nmr studies confirm 
 that the pbzma block becomes partially plasticized in hot d26-dodecane , which must cause a subtle shift 
 in the relative volume fractions of the stabilizer and core - forming 
 blocks . because the worm phase occupies only a narrow phase region , 
 this in turn induces a morphological transition . however , these observations 
 immediately suggest an apparent paradox . assuming a constant degree 
 of solvation for the plma stabilizer chains , greater solvation of 
 the core - forming pbzma block due to its plasticization would be expected 
 to increase its relative volume fraction , and so reduce the molecular curvature of the copolymer chains . this should produce 
 a worm - to - vesicle transition , rather than the worm - to - sphere transition that is actually observed ( see 
 figure 3 ) . the partial solvation of the core - forming 
 pbzma block indicated by the variable - temperature h nmr 
 studies most likely involves surface plasticization of the sterically stabilized worms . thus , the ingress of solvent 
 into the worms leads to those segments of the core - forming pbzma block 
 that are closest to the plma stabilizer chains becoming preferentially 
 solvated . this causes an increase in the effective stabilizer block 
 dp ( and a concomitant reduction in the effective core - forming block 
 dp ) and hence higher molecular curvature for the diblock copolymer 
 chains . this hypothesis is physically realistic and accounts for the 
 observed worm - to - sphere transition ; it is supported by h nmr studies of a polystyrene - based diblock copolymer in d18-octane reported by heatley and co - workers . surface plasticization can also be invoked 
 to account for the worm - to - sphere transition observed for the aqueous 
 worm gel formulation previously reported by blanazs et al . ] more recently , larue and co - workers used 
 a combined theoretical 
 and experimental approach to explore the thermally induced worm - to - sphere 
 transition exhibited by a polystyrene polyisoprene diblock 
 copolymer in n - heptane , which is a selective solvent 
 for the polyisoprene block . static light 
 scattering ( sls ) and atomic force microscopy ( afm ) were used to characterize 
 this transition , which was described as  reversible  . 
 however , although the worm - to - sphere transition induced by heating 
 from 25 to 35 c was relatively fast , the reported sls data actually 
 showed that the sphere - to - worm transition was still substantially 
 incomplete even after 36 days at 25 c . bearing in mind 
 the dls studies reported in the present work , this is almost certainly 
 because of the dilute solution conditions ( < 0.10% ) required for 
 the sls experiments . in contrast , our rheology studies summarized 
 in figure 4 were performed at 20% w / w and indicate 
 relatively good reversibility for the worm - to - sphere - to - worm transition 
 exhibited by plma16pbzma37 on much shorter 
 time scales ( minutes ) . more specifically , the cgt values observed 
 for the heating and cooling cycles are very similar , but a somewhat 
 weaker worm gel is obtained ( final g  
 10 pa , as compared to an initial g 
 of 10 pa ) . the radius of gyration of the plma 
 corona block ( rg ) was found to be 1.3 
 nm in all cases . the q range available 
 for worm contour length analysis means that a reliable upper limit 
 value can not be calculated . saxs is a powerful technique for characterizing the 
 morphologies 
 of various types of colloidal dispersions . it is statistically much more robust than tem , because 
 the x - ray scattering data are averaged over millions of particles 
 in solution . moreover , unlike sls and dls , saxs can be performed at 
 relatively high copolymer concentrations . accordingly , a series of absolute intensity saxs measurements were 
 recorded for plma16pbzma37 copolymer 
 particles at both 1.0 and 5.0% w / w . in a typical i(q ) versus q plot , the low q ( guinier ) region is particularly 
 useful for assessing the particle morphology . a gradient of zero indicates 
 a spherical morphology , whereas a negative gradient of unity is obtained 
 for rods . in the present study , the plma16pbzma37 worms are highly anisotropic but 
 relatively flexible , so a negative gradient close to ( but less than ) 
 unity is expected . previously , blanazs et al . utilized saxs to study the thermally induced worm - to - sphere transition 
 for pgma phpma diblock copolymer nanoparticles in aqueous solution 
 and observed excellent reversibility at a copolymer concentration 
 of 10% w / w . saxs patterns obtained for plma16pbzma37 diblock copolymer nano - objects in n - dodecane 
 as 
 a function of temperature are shown in figure 7a . at 20 c , the scattering pattern obtained at 5.0% w / w copolymer 
 has a negative gradient slightly lower than unity in the low q region and also a local minimum in intensity at high q ( 0.50 nm ) , indicating a mean 
 worm cross - section of approximately 13 nm . heating to 160 c 
 leads to a substantial change in this saxs pattern : the gradient at 
 low q tends to zero , and the feature at q  0.50 nm disappears . on returning to 
 20 c , a negative gradient at low q is again 
 observed ( albeit marginally lower than the original gradient ) , and 
 the minimum at q  0.50 nm is almost completely recovered . these observations suggest 
 fairly good reversibility for the worm - to - sphere - to - worm 
 transition over the experimental time scale ( hours ) . in contrast , 
 the same thermal cycle conducted at 1.0% w / w provides good evidence 
 for the same worm - to - sphere transition , albeit accompanied by a significant 
 reduction in reversibility ( as judged by the less steep gradient at 
 low q and a small but discernible shift in the local 
 minimum at high q ; see lower set of three saxs patterns 
 shown in figure 7a ) . notwithstanding 
 the above qualitative observations , detailed saxs 
 analysis requires relatively low copolymer concentrations to avoid 
 inter - particle interactions , which suppress the scattering intensity 
 at low q. in practice , the scattering 
 patterns can be satisfactorily fitted at a copolymer concentration 
 of 1.0% w / w using an established model for worm - like 
 micelles ( see the supporting information for further details ) . figure 7b shows 
 saxs data obtained for 1.0% 
 w / w plma16pbzma37 particles on heating 
 from 20 c to 160 c . at 20 c , the scattering pattern 
 can be analyzed using the aforementioned worm model . table 1 shows the key parameters obtained from the data 
 fit using this model . notably , the total worm cross - section is 15.4 
 nm and the worm contour length exceeds 600 nm , indicating highly anisotropic 
 particles with a mean aspect ratio of more than 39 ; see table 1 . another interesting observation 
 is that these plma16pbzma37 worms are 
 relatively stiff , because their kuhn length ( 160 nm ) is an order of 
 magnitude greater than the total worm cross - section . this is consistent 
 with the bottle brush - like structure of the plma stabilizer block . furthermore , both h nmr and saxs studies indicate little 
 or no solvation of the worm core - forming pbzma block at 20 c 
 ( see figure 6 and table 1 , final column ) . the radius of gyration of 1.3 nm for the coronal 
 plma block indicated by saxs is close to that estimated on the basis 
 of the mean dp of the plma block ( 1.0 nm ; see the supporting information ) . on heating to 90 c , 
 the mean worm cross - section does not vary ( although the minimum at q  0.50 nm virtually disappears ) . however , the worm contour length is reduced from more than 600 to 
 350 nm , indicating partial worm disintegration . at this elevated 
 temperature , the degree of core solvation ( xsol ) increases to 0.29 , suggesting ingress of n - dodecane into the pbzma cores ( also indicated by the h nmr studies shown in figure 6 ) . on further 
 heating to 160 c , the saxs pattern can now only be satisfactorily 
 fitted using parameters that approximate to isotropic particles ( i.e. , a total worm cross - section = 15.1 nm , worm contour 
 length = 17.3 nm , and worm kuhn length = 16.8 nm ) . this indicates 
 that the vast majority of particles in the dispersion are now actually 
 spherical micelles , with only a relatively minor population of worms 
 remaining at 160 c ( as indicated by the upturn in the scattering 
 curve at very low q ) . at this temperature , the degree 
 of solvation in the cores of these copolymer particles is relatively 
 high ( xsol = 0.48 ) , which supports our 
 hypothesis that surface plasticization of the pbzma core - forming block 
 is a key factor for the conversion of worms into spheres . ( a ) representative 
 saxs patterns recorded for 5.0 and 1.0% w / w 
 plma16pbzma37 copolymer dispersions 
 in n - dodecane recorded during a 2016020 
 c thermal cycle . the 1.0% w / w data are offset by a factor of 
 0.1 for clarity . gradients of zero and negative unity ( dashed gray 
 lines ) are also shown as a guide to the eye . ( b ) representative saxs 
 patterns ( symbols ) for the same 1.0% w / w plma16pbzma37 copolymer dispersion in n - dodecane recorded 
 during heating from 20 c to 160 c . in this case , saxs 
 patterns are offset a factor of 0.1 ( 90 c ) and 0.01 ( 160 c ) 
 for clarity , fits to the data ( solid lines ) are shown , and the results 
 of this analysis are summarized in table 1 . the inset shows all three scattering patterns plotted on the same 
 scale . in principle , the plma16pbzma37 worms 
 could either undergo  sequential budding  of spheres 
 or a series of  random worm cleavage  events ( see routes 
 a and b in figure 8) . however , the saxs observations 
 described above combined with the appearance of a minor population 
 of spheres in figure 5b suggest that the former 
 mechanism is more likely to be prevalent . schematic representation 
 of two possible mechanisms for the worm - to - sphere 
 transition that occurs on heating a dispersion of plma16pbzma37 diblock worms in n - dodecane : 
 ( a ) sequential budding and ( b ) random worm cleavage . it is emphasized that the worm - to - sphere transition 
 described herein , 
 which occurs on heating a worm dispersion in an n - alkane solvent , is wholly complementary to that previously reported 
 for aqueous worms , which form spheres on cooling . this is presumably 
 related to the well - known upper critical solution temperature ( ucst ) 
 effect for hydrophobic polymers in organic solvents ( e.g. , polystyrene 
 in cyclohexane ) , as opposed to the lower 
 critical solution temperature ( lcst ) effect that is widely reported 
 for many non - ionic water - soluble polymers in aqueous solution . finally , in view of the scalable nature of raft polymerization chemistry , such thermo - sensitive block copolymer formulations may offer potential 
 applications as viscosity modifiers ( thickeners ) or lubricants for 
 next - generation engine oils . in summary , plma  pbzma diblock copolymer spheres , worms , 
 or vesicles can be readily prepared via polymerization - induced self - assembly 
 at 20% w / w solids in n - dodecane at 70 c , provided 
 that the mean degree of polymerization of the plma stabilizer block 
 is relatively low . the worms form free - standing gels at 20 c , 
 but undergo degelation on heating via a worm - to - sphere order variable - temperature h nmr and saxs studies 
 indicate that this is the result of a subtle change in the relative 
 volume fractions occupied by the stabilizer and core - forming blocks 
 caused by surface plasticization of the core - forming block . this thermally 
 induced change in copolymer morphology is essentially irreversible 
 on an experimental time scale of hours when conducted in highly dilute 
 solution ( 0.1% w / w ) , as judged by dls and tem studies . this 
 is because the self - assembly of each worm from the fusion of multiple 
 spheres is highly inefficient under these conditions . in contrast , 
 much more reversible behavior is observed at higher copolymer concentrations 
 ( 5.020% w / w ) , as judged by h nmr spectroscopy , 
 tem , gel rheology , and saxs studies . however , the latter technique 
 indicates a significant reduction in mean contour length for the reformed 
 worms , which is consistent with the reduced gel strength observed 
 for the reconstituted worm gel as judged by gel rheology .
benzyl methacrylate ( bzma ) is polymerized 
 using a poly(lauryl methacrylate ) 
 macromolecular chain transfer agent ( plma macro - cta ) using reversible 
 addition  fragmentation chain transfer ( raft ) polymerization 
 at 70 c in n - dodecane . 
 this choice of solvent 
 leads to an efficient dispersion polymerization , with polymerization - induced 
 self - assembly ( pisa ) occurring via the growing pbzma block to produce 
 a range of plma  pbzma diblock copolymer nano - objects , including 
 spheres , worms , and vesicles . in the present study , particular attention 
 is paid to the worm phase , which forms soft free - standing gels at 
 20 c due to multiple inter - worm contacts . 
 such worm gels exhibit 
 thermo - responsive behavior : heating above 50 c causes degelation 
 due to the onset of a worm - to - sphere transition . 
 degelation occurs 
 because isotropic spheres interact with each other much less efficiently 
 than the highly anisotropic worms . 
 this worm - to - sphere thermal transition 
 is essentially irreversible on heating a dilute solution ( 0.10% w / w ) 
 but is more or less reversible on heating a more concentrated dispersion 
 ( 20% w / w ) . 
 the relatively low volatility of n - dodecane 
 facilitates variable - temperature rheological studies , which are consistent 
 with eventual reconstitution of the worm phase on cooling to 20 c . 
 variable - temperature 1h nmr studies conducted in d26-dodecane confirm partial solvation of the 
 pbzma block at elevated temperature : surface plasticization of the 
 worm cores is invoked to account for the observed change in morphology , 
 because this is sufficient to increase the copolymer curvature and 
 hence induce a worm - to - sphere transition . 
 small - angle x - ray scattering 
 and tem are used to investigate the structural changes that occur 
 during the worm - to - sphere - to - worm thermal cycle ; experiments conducted 
 at 1.0 and 5.0% w / w demonstrate the concentration - dependent ( ir)reversibility 
 of these morphological transitions .