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Organic monomeric glass formation by substituted ethylenediamine
ELSEVIER
Journal of Non-CrystallineSolids 191(1995) 293-303
Organic monomeric glass formation by substituted ethylenediamine T.E. Karis *, S.J. Kim, P.L. Gendler, Y.Y. Cheng IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 951204099,
USA
Received 31 October 1994; revised manuscript received 17 April 1995
Abstract Dibenzoyl N-alkyl ethylene diamine compounds were prepared and characterized to explore the effects of rotational isomerism and hydrogen bonding on organic monomeric glass formation. The compounds were R,CONHCH,CH2N(R)COR, with R = -CH, or -CH,CH, and R, = phenyl, 4-nitrophenyl, or 3,Sdinitrophenyl. Rotation takes place about the amide bonds, and the R substituent provides a steric barrier to rotation. Intermolecular hydrogen bonding takes place between the co . . H-N and NO . . H-N groups which suppresses nucleation when R, is nitro substituted. The undercooled melt viscosity and thermodynamic properties were used to compare the observed crystallization rate with that calculated from classical nucleation and growth theory. For R, = phenyl, the crystallization rate is close to that expected from the classical theory. For R, = 4-nitrophenyl or 3,5_dinitrophenyl, nucleation is suppressed through stabilization of rotational isomers by the network of hydrogen bonds in the quiescent state. The undercooled liquid of R, = 4-nitrophenyl exhibited rapid shear-induced crystallization during viscosity measurement. This is interpreted as the result of coupling between rotational isomers and the external flow field through the network of hydrogen bonding. An additional frequency factor that takes into account the interaction between rotational isomerism and the external network through hydrogen bonding is needed to modify the pre-exponential term in the nucleation rate equation.
1. Introduction Monomeric organic glass is potentially useful as a dye or pigment carrier in printing technology. This study was carried out to identify factors controlling the stability of the undercooled organic monomer melt. Previous studies categorized glass-forming organic monomers and derived correlations between their glass-forming tendencies, thermodynamic properties, and melt viscosities. Understanding the relationship between undercooled melt stability and or-
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ganic molecular structure is in the early stages of development., One approach to modeling glass formation from an undercooled melt is the kinetic treatment [l]. Within this treatment, both nucleation and crystal growth are taken into account. The crystal nucleation and growth rate are controlled by the collision frequency and the thermodynamic driving force is a function of the surface energy at the liquid/crystal interface and the free energy of crystal formation. The nucleation and growth rates are combined to obtain the volume fraction of crystals present in the undercooled melt as a function of time. The kinetic theory provides a quantitative value for the crys-
294
T.E. Karis et al. /Journal
of Non-Crystalline Solids 191 (1995) 293-303
tallinity based on measured thermodynamic properties, melt viscosity and molecular diameter. A qualitative description of glass formation is provided by their classification as being more or less strong or fragile with respect to the temperature dependence of the undercooled melt viscosity [2]. Strong liquids possess a nearly Arrhenius temperature dependence while increasingly fragile liquids show increasing non-Arrhenius temperature dependence. The two-step calorimetric method was developed to independently measure the nucleation and growth contributions to the crystallization process [3,4]. This is done by annealing the undercooled melt for various amounts of time near the glass transition temperature followed by holding at a higher temperature (below the melting point) to observe the growth of nuclei formed during the annealing. In this study we explore the systematic variation of a molecular structure which exhibits both rotational isomerism and hydrogen bonding. The observed crystallization kinetics are compared with that expected from classical nucleation and growth theory. This study is the first in which the combined effects of hydrogen bonding and rotational isomerism on stability of the undercooled liquids is investigated. Several dibenzoyl N-alkyl ethylenediamine (EDA) compounds were prepared. There is internal rotation about the amide bond, with a barrier to rotation provided by the N-alkyl substituent on one of the nitrogens [5]. The results of this study indicate that an additional pre-exponential frequency factor is needed in the kinetic equation for nucleation to take into account the interaction between rotational isomers and the external network through hydrogen bonding.
2. Experimental 2.1.
Materials
The organic monomers were of the general structure R,CONHCH2CH,N(R)COR, with R = -CH, or -CH,CH, and R, = phenyl, 4-nitrophenyl, or 3,5dinitrophenyl. Synthesis was by standard methods, and details are given in the Appendix. The molecular structures are shown in Fig. 1.
4N
a-
~>N-cH,-cH~-N$Q-N~,
B
(b)
it
(c) NO,
N4
Fig. 1. Chemical structure of the substituted EDA compounds with R = -CH, or -CH,CH,: (a) R, = phenyl, (b) R, = 4nitrophenyl, (cl R, = 3,Sdinitrophenyl.
2.2. Thermal analysis The steady-state shear viscosity of the supercooled liquids was measured on a constant stress rheometer (Car-rimed CSWOO). A 2 cm diameter, 1” cone angle, melt fixture with a 28 l.r,rn gap was employed to minimize the required sample size ( 2: 200 mg). Samples were first thoroughly melted at least 20°C above their melting temperature, T,, for 5 min. A steady torque between 10 and 500000 dyn cm was applied at each temperature. The torque level was adjusted to produce the rate of angular rotation between 0.04 and 100 1 s-l. The torque was applied for 30 s, which was sufficient to achieve steady-state rotation. The viscosity, q, was calculated from the linear slope of the shear compliance as a function of time at each temperature. The temperature was decreased incrementally from high to low until the sample was too viscous for measurable angular displacement to take place (= 0.5%) within 30 s. On the timescale of the current measurements, the fluids were Newtonian. The viscosity as a function of temperature for the compounds with R = -CH, is shown in Fig. 2 and listed in Table 1. The R, = 4nitrophenyl form of both R = -CH, and R =
T.E. Karis et al. /Journal
of Non-Crystalline Solids 191 (1995) 293-303
Table 1 Viscosity of R,CONHCH,CH2N(R)COR, units of Pa s
1
I
I
50
I
I
100 Trmperature
I
I
150
I
I
with R = -CH,
T (“C)
R, = phenyl
4-nitrophenyl
3,5-dinitrophenyl
200 195 190 185 175 170 165 160 155 150
_ _ 2.3x10-’ 3.0x 10-2
1.6x1O-2 1.7 x 10-2 2.3~ lo-* 2.8x10-* 4.3x 10-2 5.2x 1O-2 6.7X 10-2 crystals crystals crystals
_ 3.0x10-’ 9.8X10-l -
145 140 130 120 110 100 90 80 70 60 50 40 30
3.1x10-2 4.4x 10-2 5.8X 1O-2 9.5x10-2 2.3x10-l 5.5x10-l 1.6 7.0 4.5 x 10’ 5.2 X lo2 1.5 x 104 4.9 x 105
crystals crystals crystals crystals crystals crystals crystals crystals crystals crystals crystals crystals crystals
3.7 7.5 2.6 X 10’ 1.3 x 102 8.9 X 10’ 8.1 x lo3 7.0 x lo4 _ _ _ _ -
10-2
0
295
I
200
(‘C)
Fig. 2. The viscosity of the EDA compounds with R = -CH,.
-CH,CH, rapidly crystallized at 160°C during shear flow despite repeated attempts at measurement. The thermodynamic properties were measured by differential scanning calorimetry (DSC) in either an increasing temperature sweep mode or isothermally, while recording the heat flow using a DuPont 2100.
Temperature
in
3. Results Typical DSC heating curves are shown in Fig. 3. These curves are used to determine the glass transi-
(‘C)
Fig. 3. The DSC heating curves of the EDA compounds with R = -CH, and RI = 3,5-dinotrophenyl.
T.E. Karis et al. /Journal
296 Table 2 Heat capacity,
Cp (kJ/mol
of Non-Crystalline
K), of R,CONHCH,CH,N(R)COR,
Solids 191 (1995) 293-303
with R = -CHs
RI
Crystal
Liquid measured
calculated
phenyl 4-nitrophenyl 3,5-dinitrophenyl
0.472 (50-100) 0.622 (130-180) 0.748 (150-200)
0.601(80-130) 0.824 (W-210) 0.915 (180-230)
0.741(100) 1.10 (180) 1.38 (200)
Values in parentheses
indicate temperature
range W.
tion temperature, Z’s,crystal melting temperature, T,, and the enthalpy of fusion at the melting point, AH,. The first heating is done on the dried compound following recrystallization from solution. This measurement shows a crystalline melting transition as the large endotherm in Fig. 3 during the first heating at lO”C/min. The temperature of this endotherm is T,, and the total heat flow per gram of sample during the transition is multiplied by the molecular weight to get AH,. After the first heating above the melting temperature, the sample is cooled and reheated at lO”C/min. This curve (upper trace in Fig. 3) shows a transition in the heat flow at Z”. The heat capacities of the crystal and liquid listed m Table 2 were also measured using DSC. The heat
Table 3 Physical properties -CH,
of R,CONHCH,CH,N(R)COR,
with R =
Physical
RI
property
Phenyl
4-nitrophenyl
3,5-dinitrophenyl
M (g/mol): c (atoms/molecule): Tg (“Cl: T,, (“C): T;/T,: T, 3cR (kJ/mol): KH,,, &J/mol): AS, /R: AC”n (k.J/mol K):
282 39 16 118 0.74 281 31 9.5 0.13 1.3 1.6 37
372 43 59 197 0.71 356 41 10.5 0.20 1.3 2.3 67
460 47 84 226 0.71 418 60 14.5 0.17 1.2 1.4 60
1.0 4.9*0.05 9 329
1.0 4.6 f 0.2 33*3 292
II
c;ic$
AC;/AS,: ACp” Tg (hJ/mol):
WF uiscosity coefficients A X 10’ (Pa s): 1.0 4.9+0.3 D: -20*2 T,, (“C): DACFTg &.I/mol): 183
flow was carefully referenced to that for the apparatus with the sample pan and standard sapphire. The liquid heat capacities were also calculated using the method of group contributions [6] for comparison with those measured. T,, Tg, and AH,,, for the R = -CH, compounds are listed in Table 3, and those for the R = -CH,CH, compounds are listed in Table 4. Typical isothermal DSC heat flow curves following rapid cooling from above T,,, to intermediate temperatures between T, and Tg are shown in Fig. 4. The crystallization peak time is the time until the occurrence of the maximum in the endotherm during this test. This point corresponds to approximately 45% crystallinity in the sample [3]. Peak times for samples that crystallized are shown by the filled symbols in Fig. 5. Hot stage optical microscopy was also done to verify the results from isothermal calorimetry and to measure the crystal growth rate. The undercooled melt was seeded by previously formed crystalline powders. Samples of the R = -CH, compounds with R, = 4-nitrophenyl were held at various temperatures below their melting point,
Table 4 Physical -CH,CH,
properties
of R,CONHCH,CH,N(R)COR,
Physical
RI
property
Phenyl
4-nitrophenyl
M (g/mol): c (atoms/molecule): Tg (“C): T, (“C): Ts /T,: Tg 3cR (kJ/mol): AH, (kJ/mol): AS, /R:
296 42 10 25 0.71 297 22 6.6
386 46 45 170 0.72 365 37 10.0
with R =
T.E. Karis et al. /Journal
of Non-Crystalline Solids 191 (1995) 293-303 160
seeded, and the advancement of the crystal-liquid interface was recorded. The rate of interface movement was determined using an image analysis system. The crystal growth rate measured as a function of temperature is shown in Fig. 6.
,
0
; -
X=1--exp(-SIU3td)
(1)
5i,::-_::
,
,
I
;
I;
,
,
,
,
,
,
,
,
,
,
;
1
;
;
;
;
;
;
/
- (4
4. Theory One way to describe the tendency of a melt to form a glass as it cools, and to study the effect of molecular structure on the stability of the undercooled melt, is to examine the time-temperature transformation (TlT) curve for each material. These are measured experimentally using the isothermal calorimetry technique and hot stage microscopy. The ‘ITT curves can, in principle, be calculated theoretically from classical nucleation and growth theory. The following approximate treatment was adopted from Uhlmann [l]. When a liquid is cooled below T,, the volume fraction crystallized, X, at a given temperature after time, t, is given by [7]
,
297
b) _______~___
t'l60-
e
J_
c
601 0
\
”
4
’
” 8
" 12
t Tm
#=0.2 ___----/--f ‘.._
-
_ :-_-:-z-z-z-,
_
,
J
1 '1 '1 16 20 24 Log10(Timr,.srconds)
11 1 20
Fig. 5. Time-temperature transformation (‘ITT) curves of the EDA compounds. Smooth curves are calculated from the classical nucleation and growth theory with R = -CH,. Arrows show the duration of tests that were stopped before crystallization took place: (a) squares are measured with R = -CH,, circles are for R= -CH,CH,, both with R,=phenyl, (b) R= -CH, and R, = 4-nitrophenyl, and (c) R = -CH, and R, = 3,5dinitrophenyl.
in which I is the nucleation rate and u is the crystal growth rate. The steady-state nucleation rate is given by [71 Z=Nu
II 0
1
10000
11
1
20000
I
30000
I
II 40000
Tlme(seconds)
Fig. 4. Isothermal and R, = phenyl.
DSC of the EDA compounds
with R = -CH,
exp[ _i g)&],
(4
where N is the number, of formula units of the crystallizing component phase per unit volume of the liquid, and v is the frequency of Brownian motion in the melt. This term is ‘often approximated by the Stokes-Einstein expression Y= kT/3m a3v, where k is the Boltzmann constant, a is the molecular diameter and 7 is the viscosity. This approximation is valid for a molecularly simple liquid where the
T.E. Karis et al. /Journal
298
of Non-Crystalline
activation energy for viscous flow is the same as that for diffusive motion. The surface energy, (T, is estimated from the heat of fusion. According to Tumbull’s work [S], the molar interfacial energy, oM, is approximated by uM
=
Q,,~J
Solids 191 (1995) 293-303
The crystal growth rate by a surface nucleation mechanism (large crystal case) is given by [ 1 - exp( AG/RT)]
(3)
2’3
(6)
is referred to as the Tumbull ratio. The surface energy is then t#~
02:N;1/3V-2/30M_
(4)
N,, is Avogadro’s number and V is the molar volume of the crystal. The free energy change, AG, on crystallization below T, is given approximately by AG=-
AHIn(T, - T) T, (T,-T)-In
( )I. $
AC: is the heat capacity difference between liquid and crystal. Since AS/R > 4, the surface nucleation crystal growth mechanism is expected in these liquids [9,10].
N, is the number of atoms per unit area at the interface and r(4/3) is the gamma function [l]. The viscosity of an undercooled melt is described by the Vogel-Tamman-Fulcher (VTF) equation [11,12]: q=Aexp
DTO
[ T-T,
1
(7)
in which the coefficients A, D, To are determined by a non-linear regression fit to q as a function of T (smooth curves in Fig. 2). The above equations were employed to calculate the time to obtain 50% crystallinity at temperature T using the physical properties of the EDA compounds: T, = melting temperature; p = molecular weight; AH,,, = heat of fusion; AC: = heat capacity difference; A, D, T,, are viscosity coefficients; 4 = Tumbull ratio. The calculated TIT curves (Fig. 5)
Temperature
(‘C)
Fig. 6. Crystal growth rate of EDA compound with R = -CH, and R, = 4-nitrophenyl.
T.E. Karis et al. /Journal
of Non-Crystalline Solids 191 (199s) 293-303
were compared with those measured in the isothermal crystallization tests.
5. Discussion The thermodynamic properties of the EDA compounds are compared with those of other known organic glass-forming monomers in the following. The thermodynamic and flow properties of the EDA compounds are listed in Tables 3 and 4. The number of atoms/molecule was used to estimate the number of internal degrees of freedom = 3c [13]. The homologous glass transition temperature is T,/T,. For most glass-forming monomers, T,/T, is found to be 2/3 or more [13]. The range of T,/T, in this study of 0.74-0.71 is comparable with the hydrogen bonding sugar glucose with T,/T, = 0.7 and the interlocking branched ortho-terphenyl with T,/T, = 0.71 ]141. The cohesive energy density, U, can be approximately written as [13] Tg = U/3cR.
(8)
U was increased by increasing nitro group substitution, which produced the increase in ,Tg. The AH,,, also increased with nitro content. This increase reflects the intermolecular bonding energy difference between the crystal and the melt at T, and contributes to the driving force for crystallization. The entropic contribution for large crystals is given by [15]
AS, = A H,/T,,
(9)
and these are listed as A&/R in Table 3. When AS, > 4R, the growing crystal interface should be smooth on an atomic scale, and the interface is expected to be faceted [9,10]. The ASJR values for these substituted EDA compounds are larger than that for ortho-terphenyl which has A&,/R = 6.2 [15]. Ortho-terphenyl is a larger molecule with fewer degrees of freedom than the substituted EDA. The change in heat capacity between liquid and crystal, AC:, was determined from the measured DSC data listed in Table 2. This change is comparable with 0.27 kJ/mol K for glucose [16]. The reduced heat capacity, Ck/C$ is given in row 10 of Table 3. These are somewhat lower than for most
299
other organic glass-forming monomers. For example Cj/Ci is approximately 1.5 for glucose 1161. The conformational contribution to melting is lower for diamides than for linear hydrocarbons with the same number of conformationally flexible chain bonds due to reduction in conformational freedom of the chain segments by hydrogen bonding [17]. The relative rate of entropy change with temperature is AC,/AS, [16]. The value of ACJAS, for the EDA compounds is typical that for hydrogen bonding materials. For hydrogen bonding sugars, ACJAS, is 1.1-2.2 1161. The quantity, AC: T,, is related to a hyperbolic dependence of ACp on T for several organic glassforming systems [Ml. Here, ACFT, is largest for the R, = 4-nitrophenyl derivative. ACpTp also plays a role in determining the temperature dependence of the viscosity, as discussed below. The VTF equation (7) was employed to fit the viscosity temperature data in Fig. 2. There is limited data for the R, = 4-nitrophenyl derivative because it crystallized when cooled below 160°C during shear. The pre-exponential coefficient, A, is related to the limit of l/T + 0 by [2] A=~(T+~)=GJ~.
(10)
Here G, is the limiting high-frequency shear modulus and TV is the limiting high-temperature shear relaxation time. The data in the literature show that A = 10e5 Pa s for low-molecular-weight glass-formers, e.g., Ref. [2]. Consequently, the value of A was fixed during non-linear regression fit of the VTF equation. To is the temperature at which the free volume disappears or, alternatively, the temperature where the configurational entropy, S, + 0. In general, To = Tg - 50”. Within experimental error (90% confidence interval shown in Table 31, the VTF coefficient, D = 4.6-4.9 for the EDA compounds, and coefficient D is proportional to the ratio of a kinetic to a thermodynamic contribution [111: D a Ap/ACFTg,
(11)
where AF is a barrier crossing energy such as the breaking of hydrogen bonds during cooperative rearrangement. DACFTg a Ap is given in the last row of Table 3. Ap is highest for R, = 4-nitrophenyl.
300
T.E. Karis et al. /Journal
of Non-Crystalline
The coefficient D given by Eq. (11) is the ratio of energy barrier heights to the density of energy minima [12]. Since these energy terms are a complicated function of molecular structure and hydrogen bonding, one cannot a priori predict the effects of nitro substitution on A/L. Within the classification of strong and fragile liquids according to their D value, the EDA compounds lie within the region of fragile liquids. The properties of these EDA compounds are comparable with other well-known glass-forming monomers. The distinguishing feature of the compounds was discovered when comparison was made with the classical nucleation and growth theory. The properties listed of the EDA compounds with R = -CH, were employed to calculate the ‘ITT diagrams indicated by the smooth curves in Fig. 5. Here, the surface energy was approximated using Tumbull ratios of 0.15 and 0.2. The calculated crystal growth rate of R, = 4-nitrophenyl using a Tumbull ratio of = 0.15 resulted in reasonably good agreement with the experimental growth rate (Fig. 6). In Fig. 5(a), the filled symbols are from the isothermal crystallization measurements. The squares were measured using the EDA compounds with R, = -CH,, and the circles were measured using the EDA compounds with R, = R, = -CH,CH,, phenyl. The bar in Fig. 5(a) indicates the duration of a typical crystallization exotherm. The right-pointing arrows in Fig. 5 indicate the temperature and time duration of tests that were stopped before the crystallization exotherm. The upper and lower horizontal dashed lines in Fig. 5 indicate T, and Ts, respectively, for the EDA compounds with R, = -CH,. The calculated TIT curves predict little difference between the various EDA compounds (within an order of magnitude). However the experiments from both the DSC and the hot stage optical microscopy showed significant differences between R, = phenyl and R, = 4-nitrophenyl or 3,5-nitrophenyl. The R, = phenyl derivative was fully crystallized at about the nose temperature (50°C) within 2.5 h while the R, = phenyl and R, = 4-nitrophenyl and 3,5nitrophenyl compounds did not crystallize even after 13 days near the nose temperature. By contrast with the absence of crystallization during the isothermal tests, the growth rate of the nitro-substituted compounds was in good agreement with that calculated
Solids 191 (I 995) 293-303
from Eq. (61, as shown in Fig. 6. The suppression of crystallization is attributed to suppressed nucleation in the nitro-substituted compounds. The suppression of nucleation in the nitro-substituted compounds is related to their mokcular structure. The kinetic parameter, V, may need to be modified to take into account the presence of rotational isomers coupled by hydrogen bonding. If the timescale of the internal rotation, rir, is much smaller than the timescale of diffusive motion, rdm, the probability of contact between the same type of isomer is higher than it is when internal rotation is hindered by an external network of hydrogen bonding (rir X- Q,,,). Nucleation in the undercooled melts could be investigated further using the two-step method. Another important observation was that both the R = -CH, and R = -CH,CH, EDA compounds with R, = 4-nitrophenyl crystallized rapidly upon cooling to 160°C while being subjected to shear during the viscosity measurements. However, when heated above the melting point then rapidly cooled to and held in a quiescent state at lOO”C, this material did not crystallize even after 13 days. Strain-induced crystallization of polymers is well known. The thermodynamic driving force for strain-induced crystallization decreases the melt entropy due to orientation. This decrease increases the free energy driving force for nucleation [ 191. This melt entropy reduction may contribute to the shear-induced crystallization of the R, = 4nitrophenyl compounds. However, shear-induced crystallization was absent in the R, = phenyl and R, = 3,5-dinitrophenyl compounds. This crystallization indicates that the shear-induced crystallization is related to the interaction of the rotational isomeric structures with the shear field through intramolecular hydrogen bonding. In a crystal of unsubstituted EDA, the R, = phenyl compound is linked by hydrogen bonding to four other neighbors in a two-dimensional network, and the phenyl rings are coplanar with one another in adjacent molecules [20,21]. In order for a stable nucleus to form, the two molecules in the planar configuration must combine. When the rotation about the amide bond is unrestricted by hydrogen bonding, as with R, = phenyl, the timescale for internal rotation is fast relative to diffusion, and the nucleation and growth proceeds according to the classical theory. When the phenyl is
T.E. Karis et al./Journal
of Non-Crystalline
linked to the surroundings by the nitro-substitution, free rotation is inhibited, and the nucleation process becomes limited by the probability of two species with the phenyl rings coplanar impinging on one another to form a stable nucleus. In the quiescent state of the undercooled liquid of the nitro-substituted EDA compounds, this event has a low probability of occurrence. Below T,, thermal motions of translation and rotation are increasingly slowed by intermolecular forces and are eventually frozen at Z’a. Intramolecular rotations coupled to adjacent molecules by hydrogen bonding are, correspondingly, slowed. This coupling of intramolecular rotations to the external network is also the mechanism by which external strain can influence the equilibrium distribution of isomers. The isomeric forms are schematically illustrated in Fig. 7. The forms on the left side are one of the minimum energy configurations for an isolated molecule determined using commercial molecular modeling software. The isomeric forms on the right in Fig. 7 were obtained by rotating the amide bond to
(d)
/ (f)
(e)
+f--7b i\;i-
Fig. 7. Stick diagram of the substituted EDA compounds R,CONHCH,CH2N(R)COR, with RI = -CH, showing one minimum energy configuration for the isolated molecule ((a), (c) and (e)) and the approximate configuration in the molecular crystal of the compound t(b), (d) and (f)), (a) and (b) RI = phenyl, 6) and (d) R, = 4-nitrophenyl, and (e) and (f) R, = 3,5-dinitrophenyl.
Solih 191 (199.5) 293-303
301
approximately the configuration expected in a crystal. The mechanism for shear-induced crystallization of the R, = 4-nitrophenyl compounds is that the strain not only decreases the melt entropy but also shifts the population of isomers from that shown in Fig. 7(c) toward the form in Fig. 7(d) that is necessary for crystallization to occur. The location of the nitro groups on the ring in the R, = 3,5dinitrophenyl form probably interact with the shear field such that, even though the population is shifted toward another isomeric form, it is not one that forms a stable nucleus.
6. Conclusions The properties of the substituted EDA compounds are comparable with those of other organic glassforming monomers with hydrogen bonding. Attachment of the phenyl ring to the network by nitro-substitution hinders rotational isomerism which impedes the formation of a stable nucleus. This attachment suppresses nucleation in the quiescent undercooled liquid phase of the 4-nitrophenyl and 3,5-dinitrophenyl substituted monomers. Once nucleated, the crystal growth rate is normal. In order to adapt the kinetic description of undercooled liquids to organic monomers having rotational isomerism coupled to the external network through hydrogen bonding, an additional frequency factor is needed to modify the pre-exponential term in the nucleation rate equation. The authors would like to thank R. Siemens and H.D. Truong for the thermal analysis. They are also grateful for the support and guidance of A.F. Diaz and J.R. Lyerla. This work was sponsored by Lexmark, Inc., Lexington, Kentucky.
Appendix. Synthesis The amidization reaction 1221was carried out in a 100 ml round bottom three-necked flask with a thermometer, an addition funnel capped with a dry nitrogen inlet, and a stir bar. The initial charge was 42 mmoles of benzoyl, 4-nitrobenzoyl or 3,5-dinitrobenzoyl chloride in 40 ml of chloroform or
302
T.E. Karis et al. /Journal
Table 5 NMR chemical shifts for R,CONHCH,CH,N(R)COR,
of Non-Crystalline
with R = -CH,
Solids 191 (1995) 293-303
and R, = phenyl in polar and non-polar
solvent
CDCl, solvent (non-polar)
DMSO-d6 solvent (polar)
13
‘H
13C
‘H
8.664, s, broad 8.564, s, broad, total 1H 7.841, m, 2H 7.405, m, 8H 3.661, m 3.575, m, 2.5H (water) 3.378, m 3.04, s, 1.35H 2.931, s, 1.65H
170.5 166.7 136.7 134.6 134.1 131.2 129.3 128.3 128.2 127.2 126.7 49.97 46.73 37.77 37.31 36.86 32.65
7.859, d,J = 11.5 7.728, s, broad, total 3H 7.37, m, total 6.5H 3.834, m 3.766, m 3.540, s, broad, total 4H 3.109, s, 0.33H 3.038, s, 3H
C
173.1 167.6 133.9 131.3 129.8 128.4 126.9 126.8 46.78 38.71 37.86
s, singlet; m, multiple%.
methylene chloride reaction solvent. The reactant mixture consisted of 20 mmol of N-Methyl EDA and 42 mmol of triethylamine as an acid scavenger, which had been dried over KOH. This reactant mixture is added dropwise to the three-necked flask while maintaining the temperature below - 10°C using a dry ice and acetone bath. Notice that a 5% molar excess of the chloride and the acid scavenger were used to avoid mono-substituted amide products. Table 6 ‘H NMR chemical shifts for R,CONHCH,CH,N(R)COR, R = -CH, in DMSO-d6 R, 4-nitrophenyl
3,5-dinitrophenyl
9.044, pseudotriplet 8.869, s, broad, total 1H 8.325, m 8.117, m 7.65, m 7.505, m, 8H total 3.705, m 3.626, m 3.428, s, broad, total 4H 3.369, water 3.086, s 2.929, s, total 3H
9.453, 9.161, 9.067, 8.925, 8.616, 8.398, 3.727, 3.486, 3.115, 2.984,
s, singlet; m, multiplet.
m s, broad s m s s, 7H total m, broad s, 4H total s, 1.2H s, 1.7H
with
Following complete reaction (typically overnight), the result was a solution and some precipitate in the three-necked flask. Water was added to extract the triethylamine hydrochloride in an aqueous phase over an organic phase containing the products. Any precipitate present was removed by filtration, and the aqueous phase was separated from the organic phase. The organic phase was combined with the water-insoluble precipitate, and the crude product was obtained by evaporation of the organic solvent. The crude product was purified by recrystallization from ethanol with 2% water. Typical product yield is 75-85%. The composition of the products was verified by several analytical techniques. Thin layer chromatography (TLC) was run on silica gel with ethyl acetate (EtOAc) or EtOAc/hexane (l/l) as the eluent. Gas chromatography (GC) was done with a model HP5890 gas chromatograph and an HP-l methyl silicone gum column (10 m X 0.53 mm = 2.65 pm film). CHN combustion analysis was done on the products by a commercial analytical laboratory (Galbraith Laboratories, Inc., Knoxville, TN). Nuclear magnetic resonance nmr (IBM Instruments model AF250) was used to obtain the ‘H (250.13 MHz) and 13C (62.9 MHz) spectra in deuterated dimethyl sulfoxide DMSO-d6, and in deuterated
T.E. Karis et al./Journal
ofNon-Crystalline Solids 191 (1995) 293-303
chloroform CDCl, when soluble. Tetramethylsilane was used for an internal reference. The NMR peaks are listed in Tables 5 and 6. Here ‘s’ denotes a singlet and ‘m’ denotes a multiplet. The rotational isomerism appears directly in the ‘H NMR of the compound with R, = phenyl, Table 5, in the ratios of the two N-methyl signals (last two peaks in the first and third columns). From this the ratio of rotational isomers is about 1: 1.22 in polar DMSO-d, solvent and 1: 9 in the less polar CDCl, solvent. References [l] D.R. Uhlmann, J. Non-Cryst. Solids 7 (1972) 337. 121 C.A. Angell, in: Relaxation in Complex Systems, ed. K.L. Ngai and G.B. Wright (National Technical Information Service, Washington, DC, 1984) p. 3. [3] D.R. MacFarlane, R.K. Kadiyala and C.A. Angell, J. Chem. Phys. 79 (1983) 3921. [4] R.K. Kadiyala and C.A. Angell, Coll. Surf. 11 (1984) 341. [5] M.B. Robin, F.A. Bovey and H. Basch, in: The Chemistry of Amides, ed. J. Zabicky (Interscience, New York, 1970) p. 23.
303
[6] R.H. Perry and C.H. Chilton, Chemical Engineers Handbook, 5th Ed. (McGraw-Hill, New York, 1973). [7] J.W. Christian, ‘Ibe Theory of Transformations in Metals and Alloys, 2nd Ed. (Pergamon, New York, 1975). 181 D. Tumbull, J. Appl. Phys. 21 (1950) 1022. [9] K.A. Jackson, in: Liquid Metals and Solidification (American Society for Metals, Cleveland, OH, 1958). [lo] K.A. Jackson, in: Growth and Perfection of Crystals, ed. R.H. Doremis, B.W. Roberts and D. Tumbull (Wiley, New York, 1958). [ll] G. Adam and J.H. Gibbs, J. Chem. Phys. 43 (1965) 139. [12] C.A. Angell, J. Non-Cryst. Solids 131-133 (1991) 13. [13] D.R. Uhlmann and N.J. Kreidl, eds., Glass: Science and Technology, Vol. 1 (Academic Press, New York 1983). [14] A.R. Ubbelohde, The Molten State of Matter (Wiley, New York, 1973). [15] I. Gutzow, I. Avramov and K. K&tner, J. Non-Cryst. Solids 123 (1990) 97. [16] W. Kauzmann, Chem. Rev. 43 (1948) 219. [17] C. Carfagna, M. Vacate110 and P. Corradi, Thermochim. Acta 28 (1979) 265. [18] CA. Angell, Pure Appl. Chem. 63 (1991) 1387. [19] G.S.Y. Yeh and K.Z. Hong, Polym. Eng. Sci. 19 (1979) 395. [20] J. Brisson and F. Brisse, Can. J. Chem. 63 (1985) 3390. [21] P.A. Palmer and F. Brisse, Acta Crystallogr. B36 (1980) 1447. [22] J. Aspinall, J. Am. Chem. Sot. 63 (1941) 852.
Journal of Non-CrystallineSolids 191(1995) 293-303
Organic monomeric glass formation by substituted ethylenediamine T.E. Karis *, S.J. Kim, P.L. Gendler, Y.Y. Cheng IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 951204099,
USA
Received 31 October 1994; revised manuscript received 17 April 1995
Abstract Dibenzoyl N-alkyl ethylene diamine compounds were prepared and characterized to explore the effects of rotational isomerism and hydrogen bonding on organic monomeric glass formation. The compounds were R,CONHCH,CH2N(R)COR, with R = -CH, or -CH,CH, and R, = phenyl, 4-nitrophenyl, or 3,Sdinitrophenyl. Rotation takes place about the amide bonds, and the R substituent provides a steric barrier to rotation. Intermolecular hydrogen bonding takes place between the co . . H-N and NO . . H-N groups which suppresses nucleation when R, is nitro substituted. The undercooled melt viscosity and thermodynamic properties were used to compare the observed crystallization rate with that calculated from classical nucleation and growth theory. For R, = phenyl, the crystallization rate is close to that expected from the classical theory. For R, = 4-nitrophenyl or 3,5_dinitrophenyl, nucleation is suppressed through stabilization of rotational isomers by the network of hydrogen bonds in the quiescent state. The undercooled liquid of R, = 4-nitrophenyl exhibited rapid shear-induced crystallization during viscosity measurement. This is interpreted as the result of coupling between rotational isomers and the external flow field through the network of hydrogen bonding. An additional frequency factor that takes into account the interaction between rotational isomerism and the external network through hydrogen bonding is needed to modify the pre-exponential term in the nucleation rate equation.
1. Introduction Monomeric organic glass is potentially useful as a dye or pigment carrier in printing technology. This study was carried out to identify factors controlling the stability of the undercooled organic monomer melt. Previous studies categorized glass-forming organic monomers and derived correlations between their glass-forming tendencies, thermodynamic properties, and melt viscosities. Understanding the relationship between undercooled melt stability and or-
Corresponding author. Tel: + l-408 927 1619. Telefax: 408 927 3310. E-mail: [email protected]. ??
+ l-
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ganic molecular structure is in the early stages of development., One approach to modeling glass formation from an undercooled melt is the kinetic treatment [l]. Within this treatment, both nucleation and crystal growth are taken into account. The crystal nucleation and growth rate are controlled by the collision frequency and the thermodynamic driving force is a function of the surface energy at the liquid/crystal interface and the free energy of crystal formation. The nucleation and growth rates are combined to obtain the volume fraction of crystals present in the undercooled melt as a function of time. The kinetic theory provides a quantitative value for the crys-
294
T.E. Karis et al. /Journal
of Non-Crystalline Solids 191 (1995) 293-303
tallinity based on measured thermodynamic properties, melt viscosity and molecular diameter. A qualitative description of glass formation is provided by their classification as being more or less strong or fragile with respect to the temperature dependence of the undercooled melt viscosity [2]. Strong liquids possess a nearly Arrhenius temperature dependence while increasingly fragile liquids show increasing non-Arrhenius temperature dependence. The two-step calorimetric method was developed to independently measure the nucleation and growth contributions to the crystallization process [3,4]. This is done by annealing the undercooled melt for various amounts of time near the glass transition temperature followed by holding at a higher temperature (below the melting point) to observe the growth of nuclei formed during the annealing. In this study we explore the systematic variation of a molecular structure which exhibits both rotational isomerism and hydrogen bonding. The observed crystallization kinetics are compared with that expected from classical nucleation and growth theory. This study is the first in which the combined effects of hydrogen bonding and rotational isomerism on stability of the undercooled liquids is investigated. Several dibenzoyl N-alkyl ethylenediamine (EDA) compounds were prepared. There is internal rotation about the amide bond, with a barrier to rotation provided by the N-alkyl substituent on one of the nitrogens [5]. The results of this study indicate that an additional pre-exponential frequency factor is needed in the kinetic equation for nucleation to take into account the interaction between rotational isomers and the external network through hydrogen bonding.
2. Experimental 2.1.
Materials
The organic monomers were of the general structure R,CONHCH2CH,N(R)COR, with R = -CH, or -CH,CH, and R, = phenyl, 4-nitrophenyl, or 3,5dinitrophenyl. Synthesis was by standard methods, and details are given in the Appendix. The molecular structures are shown in Fig. 1.
4N
a-
~>N-cH,-cH~-N$Q-N~,
B
(b)
it
(c) NO,
N4
Fig. 1. Chemical structure of the substituted EDA compounds with R = -CH, or -CH,CH,: (a) R, = phenyl, (b) R, = 4nitrophenyl, (cl R, = 3,Sdinitrophenyl.
2.2. Thermal analysis The steady-state shear viscosity of the supercooled liquids was measured on a constant stress rheometer (Car-rimed CSWOO). A 2 cm diameter, 1” cone angle, melt fixture with a 28 l.r,rn gap was employed to minimize the required sample size ( 2: 200 mg). Samples were first thoroughly melted at least 20°C above their melting temperature, T,, for 5 min. A steady torque between 10 and 500000 dyn cm was applied at each temperature. The torque level was adjusted to produce the rate of angular rotation between 0.04 and 100 1 s-l. The torque was applied for 30 s, which was sufficient to achieve steady-state rotation. The viscosity, q, was calculated from the linear slope of the shear compliance as a function of time at each temperature. The temperature was decreased incrementally from high to low until the sample was too viscous for measurable angular displacement to take place (= 0.5%) within 30 s. On the timescale of the current measurements, the fluids were Newtonian. The viscosity as a function of temperature for the compounds with R = -CH, is shown in Fig. 2 and listed in Table 1. The R, = 4nitrophenyl form of both R = -CH, and R =
T.E. Karis et al. /Journal
of Non-Crystalline Solids 191 (1995) 293-303
Table 1 Viscosity of R,CONHCH,CH2N(R)COR, units of Pa s
1
I
I
50
I
I
100 Trmperature
I
I
150
I
I
with R = -CH,
T (“C)
R, = phenyl
4-nitrophenyl
3,5-dinitrophenyl
200 195 190 185 175 170 165 160 155 150
_ _ 2.3x10-’ 3.0x 10-2
1.6x1O-2 1.7 x 10-2 2.3~ lo-* 2.8x10-* 4.3x 10-2 5.2x 1O-2 6.7X 10-2 crystals crystals crystals
_ 3.0x10-’ 9.8X10-l -
145 140 130 120 110 100 90 80 70 60 50 40 30
3.1x10-2 4.4x 10-2 5.8X 1O-2 9.5x10-2 2.3x10-l 5.5x10-l 1.6 7.0 4.5 x 10’ 5.2 X lo2 1.5 x 104 4.9 x 105
crystals crystals crystals crystals crystals crystals crystals crystals crystals crystals crystals crystals crystals
3.7 7.5 2.6 X 10’ 1.3 x 102 8.9 X 10’ 8.1 x lo3 7.0 x lo4 _ _ _ _ -
10-2
0
295
I
200
(‘C)
Fig. 2. The viscosity of the EDA compounds with R = -CH,.
-CH,CH, rapidly crystallized at 160°C during shear flow despite repeated attempts at measurement. The thermodynamic properties were measured by differential scanning calorimetry (DSC) in either an increasing temperature sweep mode or isothermally, while recording the heat flow using a DuPont 2100.
Temperature
in
3. Results Typical DSC heating curves are shown in Fig. 3. These curves are used to determine the glass transi-
(‘C)
Fig. 3. The DSC heating curves of the EDA compounds with R = -CH, and RI = 3,5-dinotrophenyl.
T.E. Karis et al. /Journal
296 Table 2 Heat capacity,
Cp (kJ/mol
of Non-Crystalline
K), of R,CONHCH,CH,N(R)COR,
Solids 191 (1995) 293-303
with R = -CHs
RI
Crystal
Liquid measured
calculated
phenyl 4-nitrophenyl 3,5-dinitrophenyl
0.472 (50-100) 0.622 (130-180) 0.748 (150-200)
0.601(80-130) 0.824 (W-210) 0.915 (180-230)
0.741(100) 1.10 (180) 1.38 (200)
Values in parentheses
indicate temperature
range W.
tion temperature, Z’s,crystal melting temperature, T,, and the enthalpy of fusion at the melting point, AH,. The first heating is done on the dried compound following recrystallization from solution. This measurement shows a crystalline melting transition as the large endotherm in Fig. 3 during the first heating at lO”C/min. The temperature of this endotherm is T,, and the total heat flow per gram of sample during the transition is multiplied by the molecular weight to get AH,. After the first heating above the melting temperature, the sample is cooled and reheated at lO”C/min. This curve (upper trace in Fig. 3) shows a transition in the heat flow at Z”. The heat capacities of the crystal and liquid listed m Table 2 were also measured using DSC. The heat
Table 3 Physical properties -CH,
of R,CONHCH,CH,N(R)COR,
with R =
Physical
RI
property
Phenyl
4-nitrophenyl
3,5-dinitrophenyl
M (g/mol): c (atoms/molecule): Tg (“Cl: T,, (“C): T;/T,: T, 3cR (kJ/mol): KH,,, &J/mol): AS, /R: AC”n (k.J/mol K):
282 39 16 118 0.74 281 31 9.5 0.13 1.3 1.6 37
372 43 59 197 0.71 356 41 10.5 0.20 1.3 2.3 67
460 47 84 226 0.71 418 60 14.5 0.17 1.2 1.4 60
1.0 4.9*0.05 9 329
1.0 4.6 f 0.2 33*3 292
II
c;ic$
AC;/AS,: ACp” Tg (hJ/mol):
WF uiscosity coefficients A X 10’ (Pa s): 1.0 4.9+0.3 D: -20*2 T,, (“C): DACFTg &.I/mol): 183
flow was carefully referenced to that for the apparatus with the sample pan and standard sapphire. The liquid heat capacities were also calculated using the method of group contributions [6] for comparison with those measured. T,, Tg, and AH,,, for the R = -CH, compounds are listed in Table 3, and those for the R = -CH,CH, compounds are listed in Table 4. Typical isothermal DSC heat flow curves following rapid cooling from above T,,, to intermediate temperatures between T, and Tg are shown in Fig. 4. The crystallization peak time is the time until the occurrence of the maximum in the endotherm during this test. This point corresponds to approximately 45% crystallinity in the sample [3]. Peak times for samples that crystallized are shown by the filled symbols in Fig. 5. Hot stage optical microscopy was also done to verify the results from isothermal calorimetry and to measure the crystal growth rate. The undercooled melt was seeded by previously formed crystalline powders. Samples of the R = -CH, compounds with R, = 4-nitrophenyl were held at various temperatures below their melting point,
Table 4 Physical -CH,CH,
properties
of R,CONHCH,CH,N(R)COR,
Physical
RI
property
Phenyl
4-nitrophenyl
M (g/mol): c (atoms/molecule): Tg (“C): T, (“C): Ts /T,: Tg 3cR (kJ/mol): AH, (kJ/mol): AS, /R:
296 42 10 25 0.71 297 22 6.6
386 46 45 170 0.72 365 37 10.0
with R =
T.E. Karis et al. /Journal
of Non-Crystalline Solids 191 (1995) 293-303 160
seeded, and the advancement of the crystal-liquid interface was recorded. The rate of interface movement was determined using an image analysis system. The crystal growth rate measured as a function of temperature is shown in Fig. 6.
,
0
; -
X=1--exp(-SIU3td)
(1)
5i,::-_::
,
,
I
;
I;
,
,
,
,
,
,
,
,
,
,
;
1
;
;
;
;
;
;
/
- (4
4. Theory One way to describe the tendency of a melt to form a glass as it cools, and to study the effect of molecular structure on the stability of the undercooled melt, is to examine the time-temperature transformation (TlT) curve for each material. These are measured experimentally using the isothermal calorimetry technique and hot stage microscopy. The ‘ITT curves can, in principle, be calculated theoretically from classical nucleation and growth theory. The following approximate treatment was adopted from Uhlmann [l]. When a liquid is cooled below T,, the volume fraction crystallized, X, at a given temperature after time, t, is given by [7]
,
297
b) _______~___
t'l60-
e
J_
c
601 0
\
”
4
’
” 8
" 12
t Tm
#=0.2 ___----/--f ‘.._
-
_ :-_-:-z-z-z-,
_
,
J
1 '1 '1 16 20 24 Log10(Timr,.srconds)
11 1 20
Fig. 5. Time-temperature transformation (‘ITT) curves of the EDA compounds. Smooth curves are calculated from the classical nucleation and growth theory with R = -CH,. Arrows show the duration of tests that were stopped before crystallization took place: (a) squares are measured with R = -CH,, circles are for R= -CH,CH,, both with R,=phenyl, (b) R= -CH, and R, = 4-nitrophenyl, and (c) R = -CH, and R, = 3,5dinitrophenyl.
in which I is the nucleation rate and u is the crystal growth rate. The steady-state nucleation rate is given by [71 Z=Nu
II 0
1
10000
11
1
20000
I
30000
I
II 40000
Tlme(seconds)
Fig. 4. Isothermal and R, = phenyl.
DSC of the EDA compounds
with R = -CH,
exp[ _i g)&],
(4
where N is the number, of formula units of the crystallizing component phase per unit volume of the liquid, and v is the frequency of Brownian motion in the melt. This term is ‘often approximated by the Stokes-Einstein expression Y= kT/3m a3v, where k is the Boltzmann constant, a is the molecular diameter and 7 is the viscosity. This approximation is valid for a molecularly simple liquid where the
T.E. Karis et al. /Journal
298
of Non-Crystalline
activation energy for viscous flow is the same as that for diffusive motion. The surface energy, (T, is estimated from the heat of fusion. According to Tumbull’s work [S], the molar interfacial energy, oM, is approximated by uM
=
Q,,~J
Solids 191 (1995) 293-303
The crystal growth rate by a surface nucleation mechanism (large crystal case) is given by [ 1 - exp( AG/RT)]
(3)
2’3
(6)
is referred to as the Tumbull ratio. The surface energy is then t#~
02:N;1/3V-2/30M_
(4)
N,, is Avogadro’s number and V is the molar volume of the crystal. The free energy change, AG, on crystallization below T, is given approximately by AG=-
AHIn(T, - T) T, (T,-T)-In
( )I. $
AC: is the heat capacity difference between liquid and crystal. Since AS/R > 4, the surface nucleation crystal growth mechanism is expected in these liquids [9,10].
N, is the number of atoms per unit area at the interface and r(4/3) is the gamma function [l]. The viscosity of an undercooled melt is described by the Vogel-Tamman-Fulcher (VTF) equation [11,12]: q=Aexp
DTO
[ T-T,
1
(7)
in which the coefficients A, D, To are determined by a non-linear regression fit to q as a function of T (smooth curves in Fig. 2). The above equations were employed to calculate the time to obtain 50% crystallinity at temperature T using the physical properties of the EDA compounds: T, = melting temperature; p = molecular weight; AH,,, = heat of fusion; AC: = heat capacity difference; A, D, T,, are viscosity coefficients; 4 = Tumbull ratio. The calculated TIT curves (Fig. 5)
Temperature
(‘C)
Fig. 6. Crystal growth rate of EDA compound with R = -CH, and R, = 4-nitrophenyl.
T.E. Karis et al. /Journal
of Non-Crystalline Solids 191 (199s) 293-303
were compared with those measured in the isothermal crystallization tests.
5. Discussion The thermodynamic properties of the EDA compounds are compared with those of other known organic glass-forming monomers in the following. The thermodynamic and flow properties of the EDA compounds are listed in Tables 3 and 4. The number of atoms/molecule was used to estimate the number of internal degrees of freedom = 3c [13]. The homologous glass transition temperature is T,/T,. For most glass-forming monomers, T,/T, is found to be 2/3 or more [13]. The range of T,/T, in this study of 0.74-0.71 is comparable with the hydrogen bonding sugar glucose with T,/T, = 0.7 and the interlocking branched ortho-terphenyl with T,/T, = 0.71 ]141. The cohesive energy density, U, can be approximately written as [13] Tg = U/3cR.
(8)
U was increased by increasing nitro group substitution, which produced the increase in ,Tg. The AH,,, also increased with nitro content. This increase reflects the intermolecular bonding energy difference between the crystal and the melt at T, and contributes to the driving force for crystallization. The entropic contribution for large crystals is given by [15]
AS, = A H,/T,,
(9)
and these are listed as A&/R in Table 3. When AS, > 4R, the growing crystal interface should be smooth on an atomic scale, and the interface is expected to be faceted [9,10]. The ASJR values for these substituted EDA compounds are larger than that for ortho-terphenyl which has A&,/R = 6.2 [15]. Ortho-terphenyl is a larger molecule with fewer degrees of freedom than the substituted EDA. The change in heat capacity between liquid and crystal, AC:, was determined from the measured DSC data listed in Table 2. This change is comparable with 0.27 kJ/mol K for glucose [16]. The reduced heat capacity, Ck/C$ is given in row 10 of Table 3. These are somewhat lower than for most
299
other organic glass-forming monomers. For example Cj/Ci is approximately 1.5 for glucose 1161. The conformational contribution to melting is lower for diamides than for linear hydrocarbons with the same number of conformationally flexible chain bonds due to reduction in conformational freedom of the chain segments by hydrogen bonding [17]. The relative rate of entropy change with temperature is AC,/AS, [16]. The value of ACJAS, for the EDA compounds is typical that for hydrogen bonding materials. For hydrogen bonding sugars, ACJAS, is 1.1-2.2 1161. The quantity, AC: T,, is related to a hyperbolic dependence of ACp on T for several organic glassforming systems [Ml. Here, ACFT, is largest for the R, = 4-nitrophenyl derivative. ACpTp also plays a role in determining the temperature dependence of the viscosity, as discussed below. The VTF equation (7) was employed to fit the viscosity temperature data in Fig. 2. There is limited data for the R, = 4-nitrophenyl derivative because it crystallized when cooled below 160°C during shear. The pre-exponential coefficient, A, is related to the limit of l/T + 0 by [2] A=~(T+~)=GJ~.
(10)
Here G, is the limiting high-frequency shear modulus and TV is the limiting high-temperature shear relaxation time. The data in the literature show that A = 10e5 Pa s for low-molecular-weight glass-formers, e.g., Ref. [2]. Consequently, the value of A was fixed during non-linear regression fit of the VTF equation. To is the temperature at which the free volume disappears or, alternatively, the temperature where the configurational entropy, S, + 0. In general, To = Tg - 50”. Within experimental error (90% confidence interval shown in Table 31, the VTF coefficient, D = 4.6-4.9 for the EDA compounds, and coefficient D is proportional to the ratio of a kinetic to a thermodynamic contribution [111: D a Ap/ACFTg,
(11)
where AF is a barrier crossing energy such as the breaking of hydrogen bonds during cooperative rearrangement. DACFTg a Ap is given in the last row of Table 3. Ap is highest for R, = 4-nitrophenyl.
300
T.E. Karis et al. /Journal
of Non-Crystalline
The coefficient D given by Eq. (11) is the ratio of energy barrier heights to the density of energy minima [12]. Since these energy terms are a complicated function of molecular structure and hydrogen bonding, one cannot a priori predict the effects of nitro substitution on A/L. Within the classification of strong and fragile liquids according to their D value, the EDA compounds lie within the region of fragile liquids. The properties of these EDA compounds are comparable with other well-known glass-forming monomers. The distinguishing feature of the compounds was discovered when comparison was made with the classical nucleation and growth theory. The properties listed of the EDA compounds with R = -CH, were employed to calculate the ‘ITT diagrams indicated by the smooth curves in Fig. 5. Here, the surface energy was approximated using Tumbull ratios of 0.15 and 0.2. The calculated crystal growth rate of R, = 4-nitrophenyl using a Tumbull ratio of = 0.15 resulted in reasonably good agreement with the experimental growth rate (Fig. 6). In Fig. 5(a), the filled symbols are from the isothermal crystallization measurements. The squares were measured using the EDA compounds with R, = -CH,, and the circles were measured using the EDA compounds with R, = R, = -CH,CH,, phenyl. The bar in Fig. 5(a) indicates the duration of a typical crystallization exotherm. The right-pointing arrows in Fig. 5 indicate the temperature and time duration of tests that were stopped before the crystallization exotherm. The upper and lower horizontal dashed lines in Fig. 5 indicate T, and Ts, respectively, for the EDA compounds with R, = -CH,. The calculated TIT curves predict little difference between the various EDA compounds (within an order of magnitude). However the experiments from both the DSC and the hot stage optical microscopy showed significant differences between R, = phenyl and R, = 4-nitrophenyl or 3,5-nitrophenyl. The R, = phenyl derivative was fully crystallized at about the nose temperature (50°C) within 2.5 h while the R, = phenyl and R, = 4-nitrophenyl and 3,5nitrophenyl compounds did not crystallize even after 13 days near the nose temperature. By contrast with the absence of crystallization during the isothermal tests, the growth rate of the nitro-substituted compounds was in good agreement with that calculated
Solids 191 (I 995) 293-303
from Eq. (61, as shown in Fig. 6. The suppression of crystallization is attributed to suppressed nucleation in the nitro-substituted compounds. The suppression of nucleation in the nitro-substituted compounds is related to their mokcular structure. The kinetic parameter, V, may need to be modified to take into account the presence of rotational isomers coupled by hydrogen bonding. If the timescale of the internal rotation, rir, is much smaller than the timescale of diffusive motion, rdm, the probability of contact between the same type of isomer is higher than it is when internal rotation is hindered by an external network of hydrogen bonding (rir X- Q,,,). Nucleation in the undercooled melts could be investigated further using the two-step method. Another important observation was that both the R = -CH, and R = -CH,CH, EDA compounds with R, = 4-nitrophenyl crystallized rapidly upon cooling to 160°C while being subjected to shear during the viscosity measurements. However, when heated above the melting point then rapidly cooled to and held in a quiescent state at lOO”C, this material did not crystallize even after 13 days. Strain-induced crystallization of polymers is well known. The thermodynamic driving force for strain-induced crystallization decreases the melt entropy due to orientation. This decrease increases the free energy driving force for nucleation [ 191. This melt entropy reduction may contribute to the shear-induced crystallization of the R, = 4nitrophenyl compounds. However, shear-induced crystallization was absent in the R, = phenyl and R, = 3,5-dinitrophenyl compounds. This crystallization indicates that the shear-induced crystallization is related to the interaction of the rotational isomeric structures with the shear field through intramolecular hydrogen bonding. In a crystal of unsubstituted EDA, the R, = phenyl compound is linked by hydrogen bonding to four other neighbors in a two-dimensional network, and the phenyl rings are coplanar with one another in adjacent molecules [20,21]. In order for a stable nucleus to form, the two molecules in the planar configuration must combine. When the rotation about the amide bond is unrestricted by hydrogen bonding, as with R, = phenyl, the timescale for internal rotation is fast relative to diffusion, and the nucleation and growth proceeds according to the classical theory. When the phenyl is
T.E. Karis et al./Journal
of Non-Crystalline
linked to the surroundings by the nitro-substitution, free rotation is inhibited, and the nucleation process becomes limited by the probability of two species with the phenyl rings coplanar impinging on one another to form a stable nucleus. In the quiescent state of the undercooled liquid of the nitro-substituted EDA compounds, this event has a low probability of occurrence. Below T,, thermal motions of translation and rotation are increasingly slowed by intermolecular forces and are eventually frozen at Z’a. Intramolecular rotations coupled to adjacent molecules by hydrogen bonding are, correspondingly, slowed. This coupling of intramolecular rotations to the external network is also the mechanism by which external strain can influence the equilibrium distribution of isomers. The isomeric forms are schematically illustrated in Fig. 7. The forms on the left side are one of the minimum energy configurations for an isolated molecule determined using commercial molecular modeling software. The isomeric forms on the right in Fig. 7 were obtained by rotating the amide bond to
(d)
/ (f)
(e)
+f--7b i\;i-
Fig. 7. Stick diagram of the substituted EDA compounds R,CONHCH,CH2N(R)COR, with RI = -CH, showing one minimum energy configuration for the isolated molecule ((a), (c) and (e)) and the approximate configuration in the molecular crystal of the compound t(b), (d) and (f)), (a) and (b) RI = phenyl, 6) and (d) R, = 4-nitrophenyl, and (e) and (f) R, = 3,5-dinitrophenyl.
Solih 191 (199.5) 293-303
301
approximately the configuration expected in a crystal. The mechanism for shear-induced crystallization of the R, = 4-nitrophenyl compounds is that the strain not only decreases the melt entropy but also shifts the population of isomers from that shown in Fig. 7(c) toward the form in Fig. 7(d) that is necessary for crystallization to occur. The location of the nitro groups on the ring in the R, = 3,5dinitrophenyl form probably interact with the shear field such that, even though the population is shifted toward another isomeric form, it is not one that forms a stable nucleus.
6. Conclusions The properties of the substituted EDA compounds are comparable with those of other organic glassforming monomers with hydrogen bonding. Attachment of the phenyl ring to the network by nitro-substitution hinders rotational isomerism which impedes the formation of a stable nucleus. This attachment suppresses nucleation in the quiescent undercooled liquid phase of the 4-nitrophenyl and 3,5-dinitrophenyl substituted monomers. Once nucleated, the crystal growth rate is normal. In order to adapt the kinetic description of undercooled liquids to organic monomers having rotational isomerism coupled to the external network through hydrogen bonding, an additional frequency factor is needed to modify the pre-exponential term in the nucleation rate equation. The authors would like to thank R. Siemens and H.D. Truong for the thermal analysis. They are also grateful for the support and guidance of A.F. Diaz and J.R. Lyerla. This work was sponsored by Lexmark, Inc., Lexington, Kentucky.
Appendix. Synthesis The amidization reaction 1221was carried out in a 100 ml round bottom three-necked flask with a thermometer, an addition funnel capped with a dry nitrogen inlet, and a stir bar. The initial charge was 42 mmoles of benzoyl, 4-nitrobenzoyl or 3,5-dinitrobenzoyl chloride in 40 ml of chloroform or
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Table 5 NMR chemical shifts for R,CONHCH,CH,N(R)COR,
of Non-Crystalline
with R = -CH,
Solids 191 (1995) 293-303
and R, = phenyl in polar and non-polar
solvent
CDCl, solvent (non-polar)
DMSO-d6 solvent (polar)
13
‘H
13C
‘H
8.664, s, broad 8.564, s, broad, total 1H 7.841, m, 2H 7.405, m, 8H 3.661, m 3.575, m, 2.5H (water) 3.378, m 3.04, s, 1.35H 2.931, s, 1.65H
170.5 166.7 136.7 134.6 134.1 131.2 129.3 128.3 128.2 127.2 126.7 49.97 46.73 37.77 37.31 36.86 32.65
7.859, d,J = 11.5 7.728, s, broad, total 3H 7.37, m, total 6.5H 3.834, m 3.766, m 3.540, s, broad, total 4H 3.109, s, 0.33H 3.038, s, 3H
C
173.1 167.6 133.9 131.3 129.8 128.4 126.9 126.8 46.78 38.71 37.86
s, singlet; m, multiple%.
methylene chloride reaction solvent. The reactant mixture consisted of 20 mmol of N-Methyl EDA and 42 mmol of triethylamine as an acid scavenger, which had been dried over KOH. This reactant mixture is added dropwise to the three-necked flask while maintaining the temperature below - 10°C using a dry ice and acetone bath. Notice that a 5% molar excess of the chloride and the acid scavenger were used to avoid mono-substituted amide products. Table 6 ‘H NMR chemical shifts for R,CONHCH,CH,N(R)COR, R = -CH, in DMSO-d6 R, 4-nitrophenyl
3,5-dinitrophenyl
9.044, pseudotriplet 8.869, s, broad, total 1H 8.325, m 8.117, m 7.65, m 7.505, m, 8H total 3.705, m 3.626, m 3.428, s, broad, total 4H 3.369, water 3.086, s 2.929, s, total 3H
9.453, 9.161, 9.067, 8.925, 8.616, 8.398, 3.727, 3.486, 3.115, 2.984,
s, singlet; m, multiplet.
m s, broad s m s s, 7H total m, broad s, 4H total s, 1.2H s, 1.7H
with
Following complete reaction (typically overnight), the result was a solution and some precipitate in the three-necked flask. Water was added to extract the triethylamine hydrochloride in an aqueous phase over an organic phase containing the products. Any precipitate present was removed by filtration, and the aqueous phase was separated from the organic phase. The organic phase was combined with the water-insoluble precipitate, and the crude product was obtained by evaporation of the organic solvent. The crude product was purified by recrystallization from ethanol with 2% water. Typical product yield is 75-85%. The composition of the products was verified by several analytical techniques. Thin layer chromatography (TLC) was run on silica gel with ethyl acetate (EtOAc) or EtOAc/hexane (l/l) as the eluent. Gas chromatography (GC) was done with a model HP5890 gas chromatograph and an HP-l methyl silicone gum column (10 m X 0.53 mm = 2.65 pm film). CHN combustion analysis was done on the products by a commercial analytical laboratory (Galbraith Laboratories, Inc., Knoxville, TN). Nuclear magnetic resonance nmr (IBM Instruments model AF250) was used to obtain the ‘H (250.13 MHz) and 13C (62.9 MHz) spectra in deuterated dimethyl sulfoxide DMSO-d6, and in deuterated
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ofNon-Crystalline Solids 191 (1995) 293-303
chloroform CDCl, when soluble. Tetramethylsilane was used for an internal reference. The NMR peaks are listed in Tables 5 and 6. Here ‘s’ denotes a singlet and ‘m’ denotes a multiplet. The rotational isomerism appears directly in the ‘H NMR of the compound with R, = phenyl, Table 5, in the ratios of the two N-methyl signals (last two peaks in the first and third columns). From this the ratio of rotational isomers is about 1: 1.22 in polar DMSO-d, solvent and 1: 9 in the less polar CDCl, solvent. References [l] D.R. Uhlmann, J. Non-Cryst. Solids 7 (1972) 337. 121 C.A. Angell, in: Relaxation in Complex Systems, ed. K.L. Ngai and G.B. Wright (National Technical Information Service, Washington, DC, 1984) p. 3. [3] D.R. MacFarlane, R.K. Kadiyala and C.A. Angell, J. Chem. Phys. 79 (1983) 3921. [4] R.K. Kadiyala and C.A. Angell, Coll. Surf. 11 (1984) 341. [5] M.B. Robin, F.A. Bovey and H. Basch, in: The Chemistry of Amides, ed. J. Zabicky (Interscience, New York, 1970) p. 23.
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[6] R.H. Perry and C.H. Chilton, Chemical Engineers Handbook, 5th Ed. (McGraw-Hill, New York, 1973). [7] J.W. Christian, ‘Ibe Theory of Transformations in Metals and Alloys, 2nd Ed. (Pergamon, New York, 1975). 181 D. Tumbull, J. Appl. Phys. 21 (1950) 1022. [9] K.A. Jackson, in: Liquid Metals and Solidification (American Society for Metals, Cleveland, OH, 1958). [lo] K.A. Jackson, in: Growth and Perfection of Crystals, ed. R.H. Doremis, B.W. Roberts and D. Tumbull (Wiley, New York, 1958). [ll] G. Adam and J.H. Gibbs, J. Chem. Phys. 43 (1965) 139. [12] C.A. Angell, J. Non-Cryst. Solids 131-133 (1991) 13. [13] D.R. Uhlmann and N.J. Kreidl, eds., Glass: Science and Technology, Vol. 1 (Academic Press, New York 1983). [14] A.R. Ubbelohde, The Molten State of Matter (Wiley, New York, 1973). [15] I. Gutzow, I. Avramov and K. K&tner, J. Non-Cryst. Solids 123 (1990) 97. [16] W. Kauzmann, Chem. Rev. 43 (1948) 219. [17] C. Carfagna, M. Vacate110 and P. Corradi, Thermochim. Acta 28 (1979) 265. [18] CA. Angell, Pure Appl. Chem. 63 (1991) 1387. [19] G.S.Y. Yeh and K.Z. Hong, Polym. Eng. Sci. 19 (1979) 395. [20] J. Brisson and F. Brisse, Can. J. Chem. 63 (1985) 3390. [21] P.A. Palmer and F. Brisse, Acta Crystallogr. B36 (1980) 1447. [22] J. Aspinall, J. Am. Chem. Sot. 63 (1941) 852.