high - pressure techniques 
 have been successfully employed to investigate 
 the polymorphism of a number of different molecular organic solids . the exploration using high - pressure techniques allows for a greater 
 range of phase space to be investigated for new polymorphs of materials . the observation of polymorphism at high pressure is an important factor 
 for the exploration of polymerization , as different three - dimensional 
 arrangements of molecules will have an effect on the structure of 
 the polymer that is produced . a number of groups have been investigating 
 the use of high pressure ( 1100 gpa ) to investigate the chemical 
 reaction of a number of systems via the crystalline state including simple systems such as ethylene , benzene , and even 
 co2 . as an example of the pressures 
 required to induce chemical reactions , chelazzi and co - workers subjected 
 ethylene to 3.0 gpa to produce polyethylene , while santoro 
 and co - workers observed the transition of co2 into an amorphous 
 nonmolecular phase between 40 and 48 gpa . many of these studies have used spectroscopic techniques to elucidate 
 the changes in structure before and after the polymerization reaction . a recent and novel extension of the work is to combine polymerization 
 with crystal engineering , the ability to manipulate intermolecular 
 interactions to create multicomponent materials . by using crystal engineering strategies , goroff s team manipulated 
 intermolecular interactions between oxalamides and diiodobutadiyne 
 to ensure that the carbon  carbon triple bonds of diiodobutadiyne 
 aligned in a specific manner so as to aid the pressure - induced polymerization 
 process . by doing so , the authors followed the polymerization process , 
 including changes to the crystalline state , via x - ray diffraction 
 due to the maintenance of long - range order in the overall structure . further work has extended the investigation of high - pressure polymerization 
 to ring systems such as l , l - lactide and carnosine . in this 
 article , we have investigated the room - temperature behavior 
 of acrylic acid ( scheme 1 ) at high pressure 
 using neutron diffraction . acrylic acid is the simplest unsaturated 
 carboxylic acid and is the precursor to poly(acrylic acid ) ( paa ) . paa is used in a wide range of research and industrial applications , 
 ranging from superabsorbent materials to drug delivery vehicles . recent studies by murli and song and our own group have investigated acrylic acid - h4 under 
 high - pressure conditions and have revealed the existence of the new 
 polymorph of pure acrylic acid by rapid compression to 3.3 gpa or 
 through the use of a solution of acrylic acid in a pressure transmitting 
 medium ( ptm ) of 4:1 methanol : ethanol ( 50% v / v ) at 0.61 gpa . the discovery 
 of a new polymorph at high pressure is an important finding , as the 
 spatial arrangement of molecules in the crystal structure may have 
 a significant effect on the structure of the polymer that is created 
 via this polymorph . in our own work , there was an indication that 
 the x - ray radiation was in fact polymerizing the material , as the 
 diffraction pattern started to deteriorate after 15 h of data collection . for this reason , we decided to investigate the changes that occur 
 to the crystal structure before the polymerization process via neutron 
 diffraction due to its noninvasive nature . herein , we present the 
 results of this study , as well as further spectroscopic analysis of 
 the polymerization product . high - pressure neutron 
 powder diffraction data were collected for acrylic acid - d4 using the pearl diffractometer at the u.k . neutron data associated with the research 
 published in this paper can be requested from the corresponding author . perdeuteration is required to avoid the large backgrounds that would 
 be observed in hydrogenous materials due to the large incoherant scattering 
 cross section of hydrogen . as a point of note , there have been a number 
 of studies that have explored the use of neutron diffraction for the 
 hydrogenous materials to negate the requirement to deuterate samples . the sample was first mixed with 20% 4:1 methanol - d4:ethanol - d6 before 
 being added dropwise using a glass capillary into a standard ti  zr 
 alloy capsule gasket filled with loosely 
 packed ground silica wool which was used to inhibit the formation 
 of large crystallites . the methanol : ethanol mixture was used as a 
 ptm to provide quasi - hydrostatic conditions during the compression . calcium fluoride was mixed with the silica wool to act as a suitable 
 pressure marker . the resulting capsule assembly was then compressed 
 within a type v3b paris - edinburgh ( p - e ) press equipped with standard profile anvils with cores fabricated from 
 zirconia toughened alumina ( zta ) . the p - e cell ram pressure was monitored 
 and controlled by means of an automated hydraulic system . time - of - flight 
 ( tof ) neutron powder diffraction data suitable for structure refinement 
 were obtained by electronically focusing the 702 individual detector 
 element spectra of the main 2 = 90 detector bank . the 
 summed pattern was then normalized with respect to the incident beam 
 monitor and the scattering from a spherical vanadium standard sample . lastly , the diffraction pattern intensities were corrected for the 
 wavelength and scattering - angle dependence of the neutron attenuation 
 by the p - e cell anvil and gasket ( ti  zr ) materials . sample 
 pressures were calculated from the refined caf2 lattice 
 parameters and the known room - temperature equation of state . structures 
 were refined in topasacademic using a z - matrix model parametrized 
 in terms of the intramolecular bond distances , angles , and torsions 
 and the molecular position and orientation . the dft - optimized structures ( see 
 below ) were used to formulate restraints which were then applied to 
 the rietveld refinements , as described in ref ( 29 ) . figure 1 shows two rietveld fits of data at 0.32 gpa ( phase i ) and 
 0.87 gpa ( phase ii ) , and the corresponding crystallographic data for 
 selected pressures is shown in table 1 . a plot 
 of the diffraction at various pressures ( figure es1 ) as well as the 
 crystallographic information for all pressures ( table es1 ) can be 
 found in the supporting information . figure 2 shows the changes in unit cell parameters and molecular 
 volume with increasing pressure . the phase transition from phase i 
 to phase ii can clearly be observed at 0.8 gpa . during the compression 
 of acrylic acid , it was noted that the sample 
 pressure continued to increase slowly after a finite change of pressure 
 before eventually stabilizing after an interval of 30 min . we have attributed this unusual behavior to the solubility of acrylic 
 acid in the ptm and that during compression there is an initial solubilization 
 of the acrylic acid before it recrystallizes back out of solution , 
 causing the creep in pressure . this effect became less pronounced 
 at higher pressures , which would be consistent with the expected decrease 
 in solubility with pressure . the rietveld refinements of phase i at 0.33 
 gpa ( upper ) and phase 
 ii at 0.87 gpa ( lower ) . geometry optimizations were 
 performed by periodic density functional theory ( dft ) using the dmol code as part of the materials 
 studio modeling suite . the dnp numerical 
 basis set was used in combination with 
 the pbe functional with the tkatchenko the unit cell 
 dimensions were held fixed at the values obtained in pawley refinements 
 of the neutron powder diffraction data described above , and coordinates 
 were allowed to optimize . convergence was defined when the maximum 
 changes in total energy , displacement , and gradient were 10 ha , 5  10  , and 2  10 ha  , respectively . the raman modes for acrylic 
 acid - d4 were calculated using the plane - wave - pseudopotential 
 castep v4.2 program as implemented in 
 materials studio , employing the generalized 
 gradient approximation ( gga ) functional pw91 using the optimized phase i structure at 0.32 gpa . norm - conserving pseudopotentials 
 optimized for gga dft methods with a basis set cutoff energy of 830 
 ev were used . lattice parameters remained fixed , but the atomic positions 
 were optimized according to the following criteria : total energy convergence 
 5  10 ev / atom , maximum force on any atom 
 0.01 ev /  , stress 0.02 gpa , and atomic displacements 5  
 10  . the unit cell 
 parameters as a function of pressure ( upper ) . the 
 molecular volume of acrylic acid as a function of pressure ( lower ) . the dotted line is representative of the phase boundary between the 
 two phases . using the optimized structures from 
 the dft calculations , the lattice energies and molecule molecule 
 interaction energies were calculated using the pixelc module in the 
 clp package by gavezotti . electron densities 
 were calculated at the mp2/6 - 31 g * * level using gaussian 09 . calculation of the energies using the refined 
 data showed a similar trend but not as smooth as those observed with 
 the optimized structures highlighting the limitations of refining 
 powder data even with the implementation of restraints ; hence , the 
 optimized structures were used . during the creation of the z - matrix , 
 bond lengths to deuterium atoms were normalized to neutron values 
 and thus these values were retained for the energy calculations . table 
 es2 ( supporting information ) provides the 
 total lattice energy as well as the breakdown of intermolecular interactions 
 into coulombic , electrostatic , dispersion , and repulsion terms . acrylic acid phase i crystallizes 
 in a layered structure , 
 with the layers perpendicular to the c - axis ( space 
 group ibam ) , where the acid groups hydrogen bond 
 via a dimer motif ( figure 3 ) ; this is the same 
 phase that is observed in the h4 system . the dimers are arranged in the layer so that the cd groups 
 of c2 and c3 are in close contact with the oxygen atoms of a neighboring 
 dimer . pixel calculations of phase i show that by far the most favorable 
 molecule  molecule interaction is , unsurprisingly , between the 
 hydrogen bonded molecules ( interaction 1 ) with the dispersive and 
 coulombic terms showing the greatest stabilizing contribution ( figure 4a ; table es2 , supporting information ) . interaction 2 represents the interaction between the central molecule 
 and molecules directly above and below ( in different layers ) where 
 the carbonyl groups of the acid are arranged in an antiparallel arrangement 
 ( figure 4b ) . this type of interaction has been 
 investigated for its potential stabilizing contribution to the crystalline 
 state by allen et al . they showed through 
 the use of the cambridge structural database ( csd ) and ab 
 initio molecular - orbital calculations that carbonyl  carbonyl 
 interactions can be comparable to medium - strength hydrogen bonds albeit 
 that they are a little weaker in this case . while the energies calculated 
 from pixel can not be broken down into individual interactions , there 
 is a significant contribution from the dispersive component in interaction 
 2 which is known to contribute to carbonyl  carbonyl interactions . interaction 4 is the second most attractive interaction displayed 
 in phase i , where neighboring dimers in the layer interact through 
 close contacts between the cd2 and cd4 with o1 and o2 . due to the 
 layered nature of the structure , this interaction will be important 
 as the acrylic acid is subjected to an applied pressure . a layer of 
 molecules lying perpendicular to the c - axis as observed 
 in phase i and phase ii of acrylic acid with the 
 molecules hydrogen bonding through a typical carboxylic acid dimer . atom colors are assigned as follows : gray , carbon ; red , oxygen ; white , 
 hydrogen . six most imporatant interactions for the acrylic 
 acid polymorphs : 
 ( a ) int . ( b ) the interlayer interactions 
 observed in phase i. ( c ) the interlayer interactions observed in phase 
 ii . similar interactions have been identified between polymorphs and 
 their descriptor kept constant despite the structural changes between 
 phases , e.g. , int . atom colors are assigned as follows : gray , carbon ; 
 red , oxygen ; white , hydrogen . on compression to 0.69 gpa , the unit cell parameters of phase 
 i 
 compress by 0.5 , 0.27 , and 2.73% for the a- , b- , and c - axis , respectively , with the 
 molecular volume reducing by 4.47% ( figure 2 ) . increasing the pressure to 0.86 gpa initiates a sluggish transition 
 to a new phase ( phase ii , p21/c ) , similar to that of acrylic acid - h4 , which is complete after 30 min . this pressure is 
 slightly higher than was originally quoted in our previous paper of 
 acrylic acid - h4 ( 0.6 gpa , ) but this may 
 be due to either the different ratios of acrylic acid to ptm that 
 was used in this study compared with our original study ( 20% v / v ptm 
 this study ; 50% v / v previous work ) , a deuteration effect , or the sample 
 environment that has been used in this study . the phase transition 
 was also monitored in a diamond anvil cell ( dac ) where the reconstructive 
 nature can clearly be observed ( figure 5 ) . the longer time for full conversion in the dac ( 60 min ) was probably 
 due to the larger crystallites involved compared with the deliberately 
 fine powder that is observed in the p - e cell during the neutron experiment ; 
 the quality of the powder was monitored indirectly through the analysis 
 of the patterns from different detector banks . over the phase 
 transition , the a - axis approximately 
 halves , the b - axis maintains its compression rate , 
 i.e. relatively unchanged , and the c - axis elongates . these changes to the unit cell result in a small change in the molecular 
 volume ( 2.9% reduction ; cf . 2.0% from 0.52 to 0.68 gpa in phase i ) , 
 but it does lead to a significant reduction in the void volume from 
 30 to 4  ( as calculated by materials 
 mercury ) . this is an 87% reduction in 
 void volume compared with a 47% reduction that is observed from 0.31 
 to 0.68 gpa in phase i. images of the i  ii transition in acrylic 
 acid - d4 observed in a diamond anvil cell 
 taken at various times 
 after a pressure increase to 0.8 gpa . ( a ) 0 min , ( b ) 12 min , ( c ) 18 min , 
 ( d ) 26 min , ( e ) 30 min , ( f ) 40 min , ( g ) 48 min , ( h ) 56 min , and ( i ) 
 60 min . the packing of molecules within 
 the layers is similar in both phases , 
 but in phase ii , the layers are puckered and their relative positions 
 with respect to neighboring layers also change . there is a 5.52 , 7.26 , 
 and 10.94% compression of the a- , b- , and c - axes , respectively , that culminates in a 
 21.41% compression of the unit cell volume from 0.86 to 7.2 gpa . the 
 bulk modulus for phase ii was determined to be 6.6(7 ) gpa with a v0 value of 371(3 )  using a 
 murnaghan equation of state ( eos ) in eosfit5.2 . the data were difficult to fit due to the compressibility 
 of the material and the lack of low pressure data or reliable ambient 
 pressure volume ( acrylic acid is liquid under ambient pressure and 
 temperature ) . these were the best values calculated from a number 
 of different eos ( birch  murnaghan , vinet and murnaghan ) . the v0 value seems reasonable given the molecular 
 volumes of the two phases are very close to one another ; hence , using 
 the value at 0.33 gpa , one obtains a v0 value of 364  . up to this pressure , acrylic acid 
 remains molecular and no polymerization occurs , consistent with the 
 previously observed onset of polymerization at 8 gpa with loss of 
 raman modes . we have also carried out 
 raman studies on the h4 form to 
 8.2 gpa , and we see little change in the spectrum except around the 
 c  c torsion ( 350 cm ) and c = c  c 
 bend ( 380 cm ) which may be signifying 
 that the polymerization process is imminent . due to the experimental setup ( zta anvils for a better signal - to - noise ) 
 and peculiar behavior of the sample , we were unable to compress the 
 sample beyond 7.21 gpa . the six most important molecular interactions 
 ( > 4 kj mol ) in phase ii are shown in figure 4 , in which 
 analogous interactions in phases i and ii are illustrated . the interactions 
 within the layers remain the same ( interactions 1 and 4 ) even though 
 neighboring dimers are now observed at a slight angle to each other 
 due to the puckering of the layer . after the change of 10 
 kj mol over the phase transition , interaction 
 1 does not vary significantly until 4.1 gpa , where the total energy 
 for the interaction starts to become more negative overall . at this 
 point , there is an increase in the rate of change of the coulombic 
 term versus centroid distance , which may account for the effect on 
 the overall energy of interaction . toward 7.2 gpa , the energy of the 
 interaction reaches a plateau , signifying that if the pressure were 
 increased any further then either the repulsion between the molecules 
 would overcome any attractive forces or the compression would be taken 
 up elsewhere in the structure . a search of the csd shows that the 
 c1c1 ( carbonyl carbon ) distance observed 
 at 7.2 gpa is at the minimum of those represented in the database , 
 reflecting , perhaps , the lower limit that the hydrogen bonded dimer 
 can be sustained before another molecular arrangement is required 
 ( figure es2 , supporting information ) . interaction 2 still possesses an antiparallel interaction in phase 
 ii despite being in a slightly different orientation , which emphasizes 
 the importance of this interaction in stabilizing the structure . as 
 further pressure is applied to the structure , the total energy remains 
 fairly constant before becoming less favorable at a value of 3.2 
  between the centroids of the molecules . in terms of the specific 
 interaction between the carbonyl groups , the distance between the 
 carbon and oxygen at this pressure is 3.15(5 )  , which is close 
 to the energy minimum observed by allen et al . for this type of interaction ( 3.04  ) . thus , while 
 the pixel calculations calculate molecule  molecule interactions , 
 the trends in the energy associated with interaction 2 are consistent 
 with previous literature detailing the interaction between carbonyl 
 groups . the total energy only starts to become more repulsive at 4.9 
  between the centroids , which equates to a contact distance 
 of 2.16  between d2 and o2 which is somewhat surprising 
 given that this interaction is within the layer and therefore should 
 have less room for compression in comparison to interactions between 
 the layers . interaction 5 , between molecules in different layers , 
 shows the greatest change during the compression , with the overall 
 energy changing by 13 kj mol . the molecules 
 are aligned so that the alkene moieties are in close proximity to 
 one another and thus will be associated with the polymerization reaction . the distance between the alkene groups compresses to 3.519  
 which is close to the optimal distance that is observed for polymerization 
 reactions using diynes and therefore 
 is close enough to initiate the reaction . plot of intercentroid 
 distance and energy for each of the six highlighted 
 interactions in phases i ( filled symbols ) and ii ( open symbols ) of 
 acrylic acid . the behavior of acrylic 
 acid on decompression was not investigated 
 using neutron diffraction ; however , the experiment was repeated using 
 raman spectroscopy and a diamond anvil cell as the pressure vessel . a few drops of the exact same solution of acrylic 
 acid - d4 in perdeuterated methanol : ethanol 
 ptm were loaded into the dac and compressed to 6.3 gpa ( a similar 
 pressure to the neutron experiment ) ( figure 7 , upper ) . this is significantly shorter than the neutron experiment ; 
 however , previous research by murli and song suggested that the high 
 pressure phase was stable for over one month at 4.5 gpa . the presence of the c = c stretch ( 1571 
 cm ) throughout the compression series shows that 
 the molecular nature is retained to 6.3 gpa , which is consistent with 
 the neutron data . this result also confirms that the energy of the 
 laser used ( 532 nm ) for pressure determination and raman spectroscopy 
 was not high enough to induce polymerization , as has been observed 
 in other molecular systems at high pressure . raman modes of the perdeuterated 
 sample were calculated using castep ( figure 7 , upper ) and show that deuteration has had a significant effect on 
 the frequencies of specific stretches ; e.g. , the c = c bond , 
 identified by murli and song , is at 1644 cm for 
 the hydrogenated sample , whereas we observe this frequency at 1571 
 cm for the d4 sample . the castep spectrum was calculated 
 using the 0.33 gpa data after geometry optimization ( upper ) ; raman 
 spectra on decompression ( lower ) . note the changes to the raman peak 
 at 1635 and 1685 cm between 2.2 and 0.2 
 gpa . on decompression , the sample remains 
 in the molecular phase to 
 2.2 gpa ; however , further decompression to 0.2 gpa shows a significant 
 relative increase in the vibrations at 1635 and 1685 cm when compared with the c = c stretch , indicating that polymerization 
 has occurred . this delay in polymerization on decompression has been 
 observed in other systems , such as benzene , where the reaction may 
 be initiated at high pressure , but it is only on the release of pressure 
 that sufficient volume is available for the reaction to reach completion . the drop from 2.0 to 0.2 gpa straddles the i  ii 
 phase transition , which would also allow freedom for the molecules 
 to move , thereby permitting polymerization to occur . we have conducted 
 subsequent experiments where we have loaded the exact same sample 
 from the neutron experiment into a diamond anvil cell , compressed 
 it to 1 gpa , and left it at this pressure for a week . even at this 
 low pressure , the material had started to polymerize which would help 
 to support our theory with respect to the polymerization mechanism . experimental 
 details for the ft - ir measurements can be found in the supporting information . upon opening the ti zr 
 gasket from the p - e cell neutron diffraction experiment , there was 
 an odor of residual unreacted acrylic acid , but the vast majority 
 was polymerized which bound together the other content of the capsule 
 ( caf2 and glass wool ) . d / o  d 
 region ( 22502750 cm ) and the o  h 
 region ( 32503750 cm ) . the o h stretch 
 will be due to water present in the sample adsorbed after opening 
 the gasket to the atmosphere due to the hygroscopic nature of poly(acrylic 
 acid ) ; paa has been used as a hydrogel in the past . the c = o stretch remains in a similar position to 
 other paa samples of different molecular weights ( purchased from sigma - aldrich , 
 figure es3 , supporting information ) ( 1700 
 cm ) ; however , the stretch does not possess a shoulder 
 toward higher wavenumbers that appears in the other higher molecular 
 weight polymers . this may be due to a greater proportion of the acid 
 moieties being engaged in hydrogen bonding ; therefore , the environments 
 around the carbonyl groups are less varied than in a polymer produced 
 through more conventional routes ; i.e. , the molecules are fully hydrogen 
 bonded in the solid state , and this is translated into the polymer 
 product . this would also affect the bands around 1178 and 1248 cm which have been characterized as c  o stretching 
 coupled with oh bending . the polymer 
 produced during the neutron experiment shows very small absorbances 
 in this region compared with the high molecular weight polymers , but 
 there is a more substantial absorbance between 800 and 1100 cm that could be partially due to the c  o stretch 
 in a highly hydrogen - bonded system but this assignment is cautious 
 as the si  o stretch from the glass wool also appears at 891 
 cm . the sample was heated at a rate 
 of 10 k min from 300 to 650 k ( figure es4 , supporting information ) . the trace shows two 
 main endothermic events at 330 and 503 k which represent dehydration 
 and the formation of poly(acrylic acid ) anhydride . these temperatures 
 are in agreement with the study by moharram and allam , indicating that , despite the observed changes 
 in the raman and ir , the thermal properties remain consistent with 
 polymers synthesized by standard procedures . we have confirmed that the acrylic acid - d4 undergoes a polymorphic transition at 0.87 gpa using 
 neutron diffraction . we have identified that acrylic acid retains 
 its molecular nature up to a static pressure of 7.21 gpa but then 
 undergoes polymerization between 0.2 and 2.2 gpa on decompression , 
 which can be attributed to the increase in molecular volume that allows 
 the polymerization reaction to proceed unhindered . we have also shown 
 that one can also initiate polymerization by applying pressures of 
 1 gpa for a period of a week . while the dsc trace shows 
 previously identified thermal events , the raman and ir present peaks 
 are different from those observed for polymers synthesized under conventional 
 polymerization conditions , thereby indicating the novel polymeric 
 structure obtained via high pressure techniques .
this 
 article details the exploration of perdeuterated acrylic acid 
 at high pressure using neutron diffraction . 
 the structural changes 
 that occur in acrylic acid - d4 are followed 
 via diffraction and rationalized using the pixel method . 
 acrylic acid 
 undergoes a reconstructive phase transition to a new phase at 0.8 
 gpa and remains molecular to 7.2 gpa before polymerizing on decompression 
 to ambient pressure . 
 the resulting product is analyzed via raman and 
 ft - ir spectroscopy and differential scanning calorimetry and found 
 to possess a different molecular structure compared with polymers 
 produced via traditional routes .